* Rebels want east Ukraine to be part of Russia
* Government has launched operation against uprising
* Plane shooting is armed forces' biggest death toll
* U.S. accuses Russia of sending in tanks (Adds Poroshenko and details pf plane, updates gas talks)
By Pavel Polityuk and Aleksandar Vasovic
KIEV/NOVOHANNIVKA, Ukraine, June 14 (Reuters) - Pro-Russian separatists shot down an army transport plane in east Ukraine on Saturday, killing 49 servicemen and dealing a blow to a military campaign to defeat the rebels and hold the country together.
President Petro Poroshenko summoned his security chiefs for consultations and promised an "adequate" response after the plane was hit by an anti-aircraft missile as it came in to land at Luhansk airport near the border with Russia
"All those involved in cynical acts of terrorism of this magnitude must be punished," he said, declaring Sunday a day of mourning for the nine crew and 40 paratroopers killed.
Charred debris was scattered for hundreds of metres over the sloping wheat field where the plane came down overnight near Novohannivka, a village 20 km (12 miles) southeast of Luhansk.
The tail section jutted up from the ground, with parts of the engines, fuselage and other parts lying around it.
A platoon of rebel forces clad in camouflage fatigues scoured through the ruins for ammunition that had been intended for the government forces in east Ukraine.
"This is how we work. The fascists can bring as many reinforcements as they want but we will do this every time. We will talk to them on our own terms, " said a stocky 50-year-old rebel who identified himself as Pyotr, using his nom de guerre.
He had an assault rifle in one hand, a light machine gun in the other and two ammunition belts round his neck.
Local residents said government forces, who hold the airport and are often described as fascists by their foes, attacked rebel positions near the airfield with jets soon after day broke.
The toll is the highest suffered by government forces in a single incident since the conflict in Ukraine began and is likely to fuel tension between Russia and Kiev's main ally, the United States, which accuses Moscow of arming the rebels.
Evidence that Russia is sending in heavy armour and weapons could encourage the United States and the EU to impose new sanctions on Moscow, so far limited largely to visa bans and asset freezes on some individuals, banks and companies.
Ukrainian forces reclaimed from rebel control the city of Mariupol on Friday, a major port for export of steel. The Ukrainian flag was raised over regional government headquarters of the city of 500,000 that had long been a focus of clashes.
Despite the continuing violence, Ukraine and Russia have begun talks on a peace plan and Moscow made a goodwill gesture by agreeing to make a last attempt to solve a gas pricing dispute before a Monday deadline to cut off supplies to Kiev.
Talks were due to resume in Kiev on Saturday evening. Cutting off supplies to Kiev could also cause disruptions to deliveries to the European Union, which gets half its gas imports from Russia through Ukraine.
The talks have been complicated by tension over the rebellion since April in east Ukraine, where separatists want Russia to absorb the Russian-speaking east following the annexation of Crimea in March.
In a sign that the separatists have increasingly powerful weaponry, they shot down a military cargo plane last week, killing three people, and a general was among 14 killed when they hit an Mi-8 transport helicopter on May 29.
They also said they had shot down a fighter jet over the city of Slaviansk on Saturday, although this was not confirmed.
Scores have been killed in the violence since April and more than 100 protesters, most of them seeking closer ties with the West, were killed in clashes with police in Kiev which led to the fall of Ukraine's Moscow-leaning president in February.
Russia fears losing influence in Ukraine which was governed from Moscow in Soviet times and is seen by Russians as the cradle of their civilisation; but it denies being behind the uprising. The rebels say they get weapons from army stockpiles.
The U.S. State Department said on Friday, however, that Russia had sent tanks, heavy weapons and rocket launchers to Ukraine in recent days in support of separatists.
The assertion that Russian tanks had been brought across the border into Ukraine is likely to deepen strains in the worst standoff between Moscow and the West since the Cold War ended.
"We assess that separatists in eastern Ukraine have acquired heavy weapons and military equipment from Russia, including Russian tanks and multiple rocket launchers," State Department spokeswoman Marie Harf said in a statement.
Harf told a briefing earlier that a convoy of three T-64 tanks, several MB-21 "or Grad" multiple rocket launchers and other military vehicles had crossed from Russia into Ukraine in the last three days.
"This is unacceptable," she said. "A failure by Russia to de-escalate the situation will lead to additional costs."
The French Foreign Ministry condemned the shooting down of the plane, calling for dialogue and saying: "The circumstances of this criminal act must be fully established and those responsible identified and brought to justice." (Additional reporting by Alessandra Prentice in Donetsk, and Guy Trompiz in Paris; Writing by Timothy Heritage; editing by Ralph Boulton)