ATME, Syria — Syrian rebels downed an army helicopter for the first time Tuesday with a ground-to-air missile from newly acquired stocks in what a watchdog said could be a turning point in the 20-month conflict.

Meanwhile Russia, which has blocked UN resolutions critical of Bashar al-Assad's regime, said it only has a "working relationship" with the Syrian president, and insisted special ties were a thing of the past.

A car bomb hit a regime security post near Damascus and clashes raged around the capital, as rebels further tightened the noose around the key northern city of Aleppo.

"It is the first time that the rebels have shot down a helicopter with a surface-to-air missile," Syrian Observatory for Human Rights director Rami Abdel Rahman said of the gunship that was on a strafing run near a besieged northwestern base.

The Sheikh Suleiman base, 25 kilometres (15 miles) west of Aleppo, is the last garrison in government hands between Syria's second city and the Turkish border.

Amateur footage posted by activists on YouTube showed rebel fighters shouting: "We hit it, God is greatest," as a helicopter plunged to the ground in a ball of flames.

The Observatory said the missile was part of a consignment newly received by the rebels that had the potential to change the balance of military power in the conflict.

Little more than a week ago, the rebels seized tanks, armoured vehicles, artillery cannons, 120-mm mortars and rocket launchers when they took the sprawling Base 46 military base, about 12 kilometres (eight miles) west of Aleppo.

The rebels, a ragtag bunch of military defectors and armed civilians, are still vastly outgunned, but analysts say they are now stretching thin the capabilities of Assad's war machine and its air supremacy by opening multiple fronts across the country.

This was evident again on Tuesday, with a car bomb killing at least two soldiers at a military police checkpoint at Jdeidet Artuz near Damascus as the regime pursued insurgents south of the capital.

Battles also raged in Moadamiyet al-Sham and nearby Daraya, where a massacre in August killed more than 500 people, according to the Observatory.

In the north, the main battleground of the conflict, rebels seized a military post 15 kilometres southeast of Aleppo, tightening the noose around the city, both the insurgents and the Observatory said.

After hours of fighting, the rebels in the area said they had taken the post at the village of Al-Mintar, near Al-Safireh.

The operation was carried out by the Islamist group Ahrar al-Sham which fights alongside but is not part of the mainstream Free Syrian Army, they said.

Seventy soldiers were killed or captured, and the rebels seized six 23-mm cannons, rocket batteries and other weapons and ammunition, they added.

It came a day after insurgents took control of a dam in an area connecting Aleppo and Raqa provinces, leaving the regime with only the highway from Damascus to send reinforcements, according to Abdel Rahman.

Elsewhere, in Idlib province in the northwest, an aerial bombardment near an olive press killed at least five people, the Observatory said.

Rebel-held Maaret al-Numan was also bombed from the sky as clashes raged at the southern entrance of the strategic town on the Damascus-Aleppo road.

Damascus insists it is fighting foreign-backed "terrorists," and state media published the names of 142 fighters from 18 countries who it said were killed alongside rebels.

Prime Minister Dmitry Medvedev of Russia, which has backed Damascus throughout the conflict, said in France that "there are no special or privileged relations with President Assad".

"Such relations... no longer exist between our country and the current president," Medvedev said. "We have had, and have, good working relations" with Damascus.

The Observatory said at least 87 people were killed across Syria on Tuesday, including 32 civilians. The watchdog has recorded a total of more than 40,000 deaths in the conflict.