AMMAN - Syrian rebels said on Sunday they had captured a helicopter base east of Damascus after an overnight assault, their latest gain in a costly battle to unseat President Bashar Assad that is drawing nearer to his seat of power.The Marj al-Sultan base, 15 km (10 miles) from the capital, is the second military facility on the outskirts of the city reported to have fallen to Assad's opponents this month.Activists said rebels had destroyed two helicopters and taken 15 prisoners."We are coming for you Bashar," a rebel shouted in an internet video of what activists said was Marj al-Sultan. Restrictions on non-state media meant it could not be verified.The rebels have been firming their hold on farmland and urban centers to the east and northeast of Damascus while a major battle has been underway for a week in the suburb of Daraya near the main highway south."We are seeing the starting signs of a rebel siege of Damascus," veteran opposition campaigner Fawaz Tello said from Berlin. "Marj al-Sultan is very near to the Damascus Airport road and to the airport itself. The rebels appear to be heading toward cutting this as well as the main northern artery to Aleppo." Assad's core forces, drawn mainly from his minority Alawite sect, an offshoot of Shi'ite Islam that has dominated power in Syria for nearly five decades, are entrenched in the capital.They also have devastating air superiority although they have failed to prevent rebels increasing their presence on the edge of the capital and in neighborhoods on the periphery.A Western diplomat following the fighting said Assad still had the upper hand. "The army will allow positions to fall here and there, but it can still easily muster the strength to drive back the rebels where it sees a danger," the diplomat said."The rebels are very short of international support and they do not have the supplies to keep up a sustained fight, especially in Damascus." Iran condemns Patriot planIran said Turkey's request to NATO to deploy Patriot defensive missiles near its border with Syria would add to problems in the region, where Iran is pitted against mostly Sunni Turkey and Gulf Sunni powers.Iran's Shi'ite rulers have stepped up support for Assad while Sunni Arab powers helped forge a new opposition coalition this month recognised by France and Britain as the sole representative of the Syrians.Syria has called the missile request "provocative", seeing it as a first step toward a no-fly zone over Syrian airspace which the opposition is seeking to help them hold territory against an enemy with overwhelming firepower from the air. Most foreign powers are reluctant to go that far.NATO has said the possible deployment of the missiles was purely defensive. The US-led Western alliance has had some talks on the request but has yet to take a decision.Turkey fears security on its border may crumble as the Syrian army fights harder against the rebels, some of whom have enjoyed sanctuary in Turkey in their 20-month-old revolt against Assad's rule.Ankara has scrambled fighter jets and returned fire after stray Syrian shells and mortar bombs from heavy fighting along the border landed in its territory.More than 120,000 Syrian refugees are sheltering in camps in southern Turkey and more are expected with winter setting in and millions of people estimated to be short of food inside Syria.Abu Mussab, a rebel operative in the area of Hajar al-Aswad in south Damascus, said the opposition fighters had given up expecting a no-fly zone. "The bet is now on better organisation and tactics," he said.The video said by activists to have been filmed at the Marj al-Sultan base showed rebel fighters carrying AK-47 rifles.An anti-aircraft gun was positioned on top of an empty bunker and a rebel commander from the Ansar al-Islam, a major Muslim rebel unit, was shown next to a helicopter."With God's help, the Marj al-Sultan airbase in eastern Ghouta has been liberated," the commander said in the video. Eastern Ghouta, a mix of agricultural land and built-up urban areas, has been a rebel stronghold for months.Damaged mobile radar stations could be seen on hilltops, with rebels waiving as they walked in the compound.Footage from Saturday evening showed rebels firing rocket-propelled grenades at the base, and what appeared to be a helicopter engulfed in flames.Last week rebels briefly captured an air defense base near the southern Damascus district of Hajar al-Aswad, seizing weapons and equipment before pulling out to avoid retaliation from Assad's air force.