* Rebels capture Tel Abyad crossing, tear down flag
* Fighters in Damascus withdraw from southern suburbs
* Amnesty says civilians main victim of government fire
* Western spies say Iran flies in weapons over Iraq
By Kadir Celikcan
AKCAKALE, Turkey, Sept 19 (Reuters) - Syrian rebels seized another border crossing with Turkey on Wednesday, consolidating their grip on a frontier through which they ferry arms for battles with President Bashar al-Assad's troops around the northern city of Aleppo.
Turkey, Assad's ally turned enemy, confirmed the fall of the Tel Abyad border post, the third of seven main crossings along the Turkish-Syrian frontier to come under rebel control - though Syrian state media spoke only of bloody fighting in the area.
In a war of slowly shifting frontlines, rebels in Damascus said they were pulling back from southern parts of the capital after weeks of bombardment of a kind condemned as a war crime by Amnesty International, which accused Assad's forces on Wednesday of targeting areas near clinics and bakeries to kill civilians.
A general who defected to the rebel side was also quoted as saying Syrian commanders had discussed using chemical weapons - a move President Barack Obama has said could prompt U.S. action.
In the latest outside intervention to try to end 18 months of conflict, the foreign minister of Iran, Assad's key regional sponsor, met the president in Damascus to discuss proposals from a four-power grouping of Iran, Egypt, Turkey and Saudi Arabia.
There was little sign of diplomatic movement, however, on a crisis in which Assad can count on Iran, to whose Shi'ite Muslim faith Assad's Alawite minority affiliates itself, as well as a sympathetic Russia; against him, the rebels are being armed by Sunni Muslim states like Saudi Arabia and receive other supplies and diplomatic support from the Western powers and Turkey.
Iranian Foreign Minister Ali Akbar Salehi, quoted by Syrian state television, assured the president of "unlimited support" in efforts to "restore peace and stability" after reforms he had made. Assad was quoted as saying he would welcome an "equitable solution that meets the interests of the Syria people".
There was no clear reference to what the four regional powers - whose interests rarely coincide - may be suggesting. Iran and Russia have resisted demands by the rebels and their allies that Assad step aside first to make way for compromise.
Syria's opposition scoff at the idea of Iran playing a role in peacemaking given its support for Assad. An intelligence report by a Western agency and seen by Reuters said Iran has used Iraqi airspace to fly in weapons and military personnel to Syria - something the Iraqi government denied, but which Baghdad's U.S. sponsors believe to be true.
Activists who collate data from across Syria said 170 people, mostly civilians, had been killed on Tuesday, a typical daily figure of late. Protests that began in March last year and were met with force have become a civil war in which more than 27,000 have died so far. The last month was the bloodiest yet.
Amnesty International said in a report that civilians, including children, are the main victims of army bombing and shelling of areas taken by the opposition. Assad's forces use "weapons which cannot be aimed at specific targets, knowing that the victims of such indiscriminate attacks are almost always civilians", said Donatella Rovera of Amnesty.
A U.N. panel has accused rebels, too, of abuses, although on a lesser scale than those committed by Assad's supporters.
At the Tel Abyad border crossing, near the Turkish town of Akcakale, 200 km (130 miles) northeast of Aleppo, rebels could be seen in television footage tearing down a Syrian flag.
"I can confirm that the border post has fallen. It is under the complete control of the rebels," a Turkish official said. Two Turkish civilians were wounded by stray bullets.
The fighting, which started on Tuesday evening, appeared to be the first move on by insurgents on the border zone in al-Raqqa province, most of which has remained solidly pro-Assad.
State-run Syria TV said: "Our heroic armed forces are chasing down terrorist remnants in the Tel Abyad region, killed a large number of them and destroyed their weapons."
Far to the south around Damascus, a rebel withdrawal from the neighbourhoods of Hajar al-Aswad, al-Asali and al-Qadam is a setback for them after gains in the capital in recent months, though fighters said the move was tactical and short-term.
"They're withdrawing to another area because we just don't have enough weapons to keep up our hit-and-run operations," said Moaz, a rebel fighter in Damascus, who was wounded last week. "The wounded need treatment and the fighters need some rest.
"The regime will move into one area and comb it for rebels, while we move to another. There are a lot of places we can go, and the fighters will be back to fight again soon."
Syrian state television accused "terrorists" of taking four electricity workers hostage before troops freed them; activists posted an Internet video showing what they said were bodies of men arrested and executed by government forces in the Damascus suburb of Jobar. It showed 11 bodies laid out in a mosque.
In Hama province, activists said helicopters dropped bombs on the village of al-Haweeja on Wednesday, killing at least eight. Video showed dust-covered civilians retrieving the crumpled bodies of the dead - at least one of them the blood- smeared corpse of a child. Screaming residents dug through piles of ruined buildings, looking for more dead and wounded.
The civilian death toll has increased pressure on Western powers to act, although the complexity and scale of the conflict are very different from that in Libya, where a limited bombing campaign allowed rebels to overthrow Muammar Gaddafi.
The head of the main opposition bloc, the Syrian National Council urged Arab and Western powers to pursue a similar course against Assad. Abdulbaset Sieda, whose exile-dominated SNC is part of a patchwork of rebel movements, told al-Hayat newspaper: "We demand everything that will stop the killing of Syrians."
However, the United States and European governments are wary of the complexities, of ethnic and sectarian rivalries coming to the fore and of weapons falling into the hands of anti-Western Islamists, who form part of the rebel forces fighting in Syria.
France, which pushed hard for NATO's Libyan campaign and is a vocal supporter of the opposition in Syria, its former colony, has "seriously" discussed arming the rebels, a diplomat said.
But Eric Chevallier, the ambassador to Syria, told French radio on Tuesday that such a move was complex. He said President Francois Hollande had asked him to work with all the opposition groups, including combatants, to help them organise themselves.
The risks of greater violence were underlined by a general who defected from Assad's army and reached Turkey three months ago: "We were in a serious discussion about the use of chemical weapons, including how we would use them and in what areas," Major-General Adnan Sillu told Britain's Times newspaper.
"We discussed this as a last resort - such as if the regime lost control of an important area such as Aleppo."
Sillu said that when he was there Syria also spoke of giving chemical weapons to the Lebanese group Hezbollah - a move that could draw in the Shi'ite movement's sworn enemy, Israel.