(Adds likelihood of more U.S. sanctions, quote from Ukrainian foreign minister)
* Setback for Kiev before four-way talks on Ukraine in Geneva
* Pro-Russian rebels drive armoured vehicles into Slaviansk
* Moscow, Kiev trade charges of "civil war" and "terrorism"
By Gabriela Baczynska and Thomas Grove
KRAMATORSK/SLAVIANSK, Ukraine, April 16 (Reuters) - S eparatists flew the Russian flag on armoured vehicles taken from the Ukrainian army on Wednesday, humiliating a Kiev government operation to recapture eastern towns controlled by pro-Moscow partisans.
Six armoured personnel carriers were driven into the rebel-held town of Slaviansk to waves and shouts of "Russia! Russia!" It was not immediately clear whether they had been captured by rebels or handed over to them by Ukrainian deserters.
Another 15 armoured troop carriers full of Ukrainian paratroops were surrounded and halted by a pro-Russian crowd at a town near an air base. They were allowed to retreat only after the soldiers handed the firing pins from their rifles to a rebel commander.
The military setback leaves Kiev looking weak on the eve of a peace conference on Thursday, when its foreign minister will meet his Russian, U.S. and European counterparts in Geneva. The meeting comes as the European Union and the United States contemplate more sanctions against Russia, with new measures from Washington possibly coming as soon as Friday.
Upon arriving in Geneva on Wednesday, Ukrainian Foreign Minister Andrii Deshchytsia said there is still time to ease tensions in Ukraine.
"I think that we still have a chance to de-escalate the situation using the diplomatic means," he said. "And we will try hard. We are trying hard - not only Ukraine - but also the United States. However, the time is now, not only to express the concerns, but to look for a more concrete and adequate response to Russia's plans and actions."
So far, the United States and European Union have imposed only targeted sanctions against a list of Russian and Ukrainian individuals and companies in retaliation for Moscow's seizure and rapid annexation of Crimea from Ukraine, completed last month.
The EU took a step towards imposing harsher sanctions on Wednesday by informing its member states of the likely impact the proposed measures would have on each of them. Countries have a week to respond before the European Commission starts drawing up plans.
As Secretary of State John Kerry flew into Geneva, a senior U.S. official said that President Barack Obama had put the onus on Moscow to calm the crisis. "With regard to sanctions, the president has been very clear that if Russia does not take this opportunity to de-escalate, the costs are going to go up," the official told reporters.
U.S. officials, speaking on the condition of anonymity, said the United States also was prepared to offer Ukraine more non-lethal assistance.
Moscow responded to the overthrow of its ally Ukrainian president Viktor Yanukovich in February by announcing its right to intervene militarily to protect Russian speakers across the former Soviet Union, a new doctrine that has overturned decades of post-Cold War diplomacy.
RUSSIAN TROOPS MASSED
Russia has massed thousands of troops near the Ukrainian frontier. A Reuters reporting team that visited the border area late last week and again on Wednesday said Russian military activity had increased markedly over the past few days, with more troops and vehicles on the roads.
A U.S. official said the Geneva meeting aimed to calm the situation in largely Russian-speaking eastern Ukraine and discuss constitutional reforms to protect the rights of minorities in the country.
"The idea here is that they (Russian leaders) would stop aiding and abetting and supporting these separatists and that they would pull their troops back from the borders," he said.
At one site in an open field in Russia's Belgorod region about 20 km (12 miles) from the frontier, 10 large army tents and about 20 military trucks were present, far more than last week, although a squadron of attack helicopters had left.
Kiev had sent the convoy of paratroops to capture an airfield, the start of an operation to reclaim towns held by separatists who have declared an independent "People's Republic" in the industrial Donbass region.
The Ukrainian government and its Western allies say Russian agents are coordinating the uprising. Moscow denies it is involved and says Kiev is precipitating civil war by sending troops to put down the revolt.
The Kiev government is seeking to reassert control without bloodshed, which it fears would precipitate a Russian invasion. Its operation is the first test of Kiev's underfunded army, which had until now had played no role in six months of internal unrest. The government seems to have resorted to using troops after losing faith that police in the east would stay loyal.
The government troops began their operation on Tuesday, arriving by helicopter to take control of an airfield at Kramatorsk. They drove armoured personnel carriers flying the Ukrainian flag into the town in the early morning.
But six of those vehicles later rumbled into Slaviansk, 15 km (9 miles) away, with Russian and separatist flags and armed men in motley combat fatigues on top. They stopped outside the separatist-occupied town hall.
Some Ukrainian troops were also taken to Slaviansk with the vehicles, although it was not immediately clear whether they had deserted or were coerced into coming. People in the town said some were sent home in buses.
One soldier guarding one of the vehicles said he was a member of Ukraine's 25th paratrooper division, the unit sent by Kiev to recapture Slaviansk and Kramatorsk.
"All the soldiers and the officers are here. We are all boys who won't shoot our own people," he said, adding that his men had had no food for four days until local residents fed them.
The Defence Ministry in Kiev said the vehicles had been captured. "A column was blocked by a crowd of local people in Kramatorsk with members of a Russian diversionary-terrorist group among them," it said. "As a result, extremists seized the equipment."
Above Slaviansk, a Ukrainian jet fighter carried out several minutes of aerobatics over the town's main square.
Back in Kramatorsk, 15 vehicles from the Ukrainian military convoy sent to recapture the town were stuck near a railroad, blockaded by unarmed local residents. A Ukrainian officer said his men were not prepared to fire on fellow Ukrainians.
"WILL NOT SHOOT AT MY OWN PEOPLE"
"I am a Ukrainian officer, that's the first thing. The other is that I will not shoot at my own people no matter what," said the officer, who declined to give his name as he was not authorised to speak to the media.
"I want things to be normal, people to go back home, not sit in some fields with weapons. I want children to see weapons only on TV ... I want us to live together as we were. And I want to be back home to my wife and child."
The crowd blockaded the troops until the commander of the unit, Colonel Oleksander Schvets, agreed to order his men to hand over the firing pins from their rifles to a separatist leader. The crowd then allowed the troops to drive back to their base in Dnipropetrovsk, a southern city.
The pro-Russian separatists began the uprising in the east by seizing government buildings in three cities on April 6, and have tightened their grip in recent days. Their armed paramilitaries now control buildings in about 10 towns and have seized hundreds of weapons. Two people were killed on Sunday in Slaviansk, including a Ukrainian state security agent shot dead.
Kiev calls the uprising a blatant repeat of the seizure of Crimea, where armed pro-Russian partisans also occupied buildings, declared independence and proclaimed themselves in charge of state bodies. The main difference so far is that Russian troops have not appeared overtly as they did in Crimea, where Moscow already had military bases.
NATO says 40,000 Russian soldiers are massed on the frontier - forces that could capture eastern Ukraine in days.
Hopes are faint for any progress at the talks in Geneva on Thursday. As in the case of Crimea last month, diplomacy appears to have fallen far behind the pace of events on the ground, with pro-Russian partisans establishing control of territory before Western countries can muster a response.
Russian President Vladimir Putin is scheduled to speak on Thursday at an annual question and answer session with citizens, which could signal how far he intends to go in Ukraine.
A triumphant speech he gave in March justifying the annexation of Crimea has been seen as a decisive moment in Russia's relations with the West, signalling Moscow no longer feels bound by customary rules governing the use of force.
Putin told German Chancellor Angela Merkel in a telephone call late on Tuesday that Kiev had "embarked on an anti-constitutional course" by using the army. "The sharp escalation of the conflict puts the country, in effect, on the brink of civil war," the Kremlin quoted him as saying.
Washington and NATO have made clear they will not fight to protect Ukraine. Instead, NATO announced urgent new steps to reinforce the security of alliance members that border on it.
"You will see deployments at sea, in the air, on land, to take place immediately. That means within days," NATO Secretary General Anders Fogh Rasmussen told a news conference after meeting of ambassadors from the 28-member alliance. (Additional reporting by Christian Lowe in Moscow, Richard Balmforth in Kiev, Nguyen Phuong Linh in Hanoi, Arshad Mohammed and Stephanie Nebehay and Catherine Koppel in Geneva; Writing by Peter Graff; Editing by David Stamp and Steve Ortlofsky)