Gunman abducted seven International Committee of the Red Cross (ICRC) and Red Crescent staff on Sunday in Idlib province, one of the main theatres of Syria's brutal war.

It came as two suicide car bombings blasted Damascus, and as the ICRC and Syrian Arab Red Crescent (SARC) evacuated thousands of people from a suburb of the capital that the army has besieged for months.

A key opposition group, meanwhile, said it would not attend any Geneva peace talks, a setback for a US-Russian proposal aimed at ending the 31-month conflict that has killed more than 115,000 people.

The aid workers -- six Red Cross staff and a SARC volunteer -- were "abducted this morning by unidentified armed men near Sareqeb," said an ICRC statement.

"We call for the immediate and unconditional release of the seven colleagues," said Magne Barth, head of the ICRC's Syria delegation.

The statement did not give the nationality of those abducted, and there has been no claim of responsibility.

The Red Cross, a rare aid group working on both sides of the conflict, said the team had travelled to Idlib on October 10 to assess the situation at health facilities and deliver aid.

"The convoy, which was on its way back to Damascus, was clearly marked with the ICRC emblem, which is not a religious symbol," it said.

Rebels control large swathes of the northwestern province of Idlib, which borders Turkey.

Kidnapping has become an increasing problem in Syria, with journalists and aid workers frequently targeted in rebel-held parts of the country, largely in the north.

Last month a German aid worker held for almost four months escaped his kidnappers in Idlib, just like his two colleagues who managed to flee in July, according to their aid group Gruenhelme.

In other violence on Sunday, two cars laden with explosives and driven by suicide bombers blew up near the state broadcaster's headquarters at night in central Damascus, state media said.

A reporter for government television made no mention of any casualties, saying only that "there were some human remains at the scene, likely those of a suicide bomber".

'World focus on murder weapon, not murderer'

On the political front, the Syrian National Council ruled out attending any Geneva peace talks, and said it would quit the umbrella opposition National Coalition if it participated.

"The Syrian National Council... has taken the firm decision... not to go to Geneva under the present circumstances (on the ground)," its chief George Sabra told AFP.

"This means that we will not stay in the Coalition if it goes" to the talks.

US Secretary of State John Kerry flew to London on Sunday, where he was to discuss the Geneva conference with Lakhdar Brahimi, the UN-Arab League envoy for Syria.

Sabra said the international community had failed to punish the regime for an August 21 sarin gas attack on Damascus' outskirts that killed hundreds of people.

Washington threatened military strikes in response to the attacks, which the United States and Syria's opposition blamed on the regime.

The strikes were averted by a US-Russian deal under which Syria is turning over its chemical arsenal for destruction.

"The international community has focused on the murder weapon, which is the chemical weapons, and left the murderer unpunished and forgotten the victims," said Sabra.

"We will not participate in a conference that is intended to hide the failure of international politics."

He also invoked the plight of Syrians in neighbourhoods besieged by regime troops, including the Damascus suburb of Moadamiyet al-Sham, where he said residents were "dying of hunger".

On Sunday, the ICRC said it and the SARC had evacuated 3,500 people from the neighbourhood within a day.

Most were women and children "in a state of major fatigue and were very scared," SARC head of operations Khaled Erksoussi told AFP.

Regime forces regularly bomb Moadamiyet al-Sham, and the opposition accuses the government of starving residents by sealing off the district, which was targeted in the August 21 sarin attack.

The government says the opposition is holding residents hostage and described the evacuation as part of its "efforts to protect citizens from terrorists".