DAMASCUS: The umbrella opposition National Coalition on Saturday condemned world powers for failing to act to stop the slaughter in Syria, as missiles killed at least 29 in second city Aleppo.

The remarks by spokesman Walid al-Bunni came after the Coalition had said it would form a government to run "liberated areas" of Syria and was pulling out of several international meetings in protest against world "silence".

"We cannot continue listening to statements that are not accompanied by action," Bunni said it remarks to France 24's Arabic-language channel.

"The world has a responsibility to protect (the Syrian people) from a butcher who has been slaughtering them for two years," a reference to President Bashar al-Assad.

Referring to a meeting of the Friends of Syria group in Rome next Thursday, Bunni said: "We want to say... if you are our real friends, help us to stop the massacres that are being committed against our people".

Late on Friday, the group had said it would not attend meetings in Italy, Russia and the United States to protest against the "shameful" lack of global condemnation of "crimes committed against the Syrian people".

It had been due to attend the Friends of Syria, and Coalition chief Ahmed Moaz al-Khatib had also been invited to Moscow.

"The international silence on the crimes committed every day against our people amounts to participating in two years of killings," a statement said.

"We hold the Russian leaders in particular ethically and politically responsible because they continue to support the (Damascus) regime with weapons."

Bunni also challenged the United States to honour what he said were promises of support for democracy in Syria.

"Our visit to Washington is on hold until Washington takes a stance that is in accordance with US statements on its support for democracy."

On Friday, Bunni announced plans for a government for "liberated areas" that he said he hoped would be based inside northern Syria.

Its composition and "prime minister" would be chosen at a meeting on March 2, he added, with Coalition members saying the gathering would be held in Istanbul.

Meanwhile, peace envoy Lakhdar Brahimi said Thursday's attack in Damascus had left about 100 people dead -- substantially more than a previous toll of 61 -- and wounded another 250.

Describing it as a "war crime", the UN-Arab League envoy said "nothing could justify such horrible actions that amount to war crimes under international law."

European Union foreign policy chief Catherine Ashton said "nothing can justify an act of such brutality that killed so many people, mostly civilians, including children."

Both the regime and opposition have blamed "terrorists" for the attack near the ruling Baath party's main offices.

The same day, another 22 people were killed in a triple bombing targeting security headquarters in northern Damascus, said the Syrian Observatory for Human Rights.

In Aleppo, at least 29 people, including 19 children, had been killed and 150 wounded when three missiles hit Tariq al-Bab district on Friday, said Observatory director Rami Abdel Rahman.

Shelling of the city's Maadi district caused a building to collapse, killing an unknown number of people, and rebels fought troops near Aleppo international airport and Nayrab air base to the southeast.

The army's use of surface-to-surface missiles is part of a bid to advance on Aleppo, swathes of which the rebels have seized since mid-2012, said Abdel Rahman.

"The army has been trying for weeks to come closer to Aleppo via its eastern entrance, in order to assault it. Elite troops are being sent... but so far the army has been unsuccessful," he told AFP.

A security official in Damascus said late last year such missiles were a Syrian-made version of Scuds, while NATO has reported the use of ballistic missiles in the country.

The Britain-based Observatory said 149 people were killed nationwide on Friday, adding to an overall UN death toll of at least 70,000 dead in the 23-month conflict.