Syrian rebels have said they had captured a helicopter base near Damascus after a battle with government forces, the second military facility on the outskirts of the capital to fall to President Bashar al-Assad's opponents this month.

An Internet video which activists said was filmed on Saturday overnight at the Marj al-Sultan base, 15km east of the capital, showed rebel fighters carrying AK-47 rifles touring the facility. An anti-aircraft gun could be seen positioned on top of an empty bunker and a rebel commander 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.

Activists said two helicopters were destroyed in the attack as well as a radar station, and that 15 personnel were taken

prisoner.

The reported takeover of the base came amid continued shelling in several areas in the outskirts of Damascus.

Helicopter gunships pounded Zamalka town, northeast of Damascus, as rebels clashed with troops in nearby Harasta, activists said.

Further advances

The opposition fighters have set up rear bases in orchards surrounding the capital, where they had made advances during the summer but have since been driven out.

In the north, rebels pressed on with an offensive against troops stationed at the strategic Tishrin dam, which straddles the Euphrates River and connects the provinces of Aleppo and al-Raqqa.

The rebels, according to a local resident, have been closing in on the area for the past week.

Opposition fighters already control one of the main routes to al-Raqqa and the dam would give them a second passage, connecting a wide expanse of territory between the two provinces, both of which border Turkey.

Last Thursday, rebels captured an army base in the eastern province of Deir Az-Zor, striking another blow against Assad's military and further weakening his control in the strategic region bordering Iraq.

In a separate development, Iran on Sunday said the deployment of Patriot defence missiles near Turkey's border with Syria would worsen tensions, as fears grow of the Syrian civil war spilling across the region.

Turkey asked NATO for the Patriot system, designed to intercept aircraft or missiles, last week after talks about how to shore up security on its 900km border.

"The installation of such systems in the region has negative effects and will intensify problems in the region," Ali Larijani, Iranian parliament speaker, said on returning from a trip to Syria, Lebanon and Turkey on Saturday evening, according to Iranian state news agency IRNA.

'Provocative request'

Ramin Mehmanparast, Iran's foreign ministry spokesperson, told the Iranian Students' News Agency (ISNA) on Sunday that deploying the Patriot system "will not only not help solve the situation in Syria, it will actually make the situation more difficult and complicated as well".

Syria has called Turkey's request for the Patriot missiles "provocative", and Russia said the move could increase risks in the conflict.

Iran has steadfastly supported Assad throughout the 20-month-old uprising against his rule.

Turkey's missile request may have riled Damascus because it could be seen as a first step toward implementing a no-fly zone over Syrian airspace.

Syrian rebels have been requesting a no-fly zone to help them hold territory against a government with overwhelming firepower from the air, but most foreign governments are reluctant to get sucked into the conflict.

Turkey fears security on its border may crumble as the Syrian army fights harder to contain the rebels, some of whom have enjoyed sanctuary in Turkey.

Heavy fighting has often erupted along Syria's border with Turkey. Ankara has scrambled fighter jets and returned fire after stray Syrian shells and mortars landed in its territory.