Syria's main opposition group said on Saturday that it was pulling out of international talks and demanded world action to stop the slaughter of Syrians, after missiles slammed into the city of Aleppo killing dozens.

The Syrian National Coalition's decided to boycott talks with world power after announcing plans to form a government to run "liberated areas" of Syria, in what analysts said was a bid to boost its credibility and win more support from the international community.

Coalition chief Ahmed Moaz al-Khatib said the group's withdrawal from meetings abroad was "a message of protest to all governments of the world, Arab and non-Arab, that can see how the Syrian people are being killed, while they merely look on."

His was speaking as the conflict, which has cost the lives of more than 70,000 people according to the UN, is set to enter its third year, with no end in sight.

Condemning the "international silence" on the bloodshed in Syria, which it charged "amounts to participating in two years of killings," the opposition said Friday that it would not attend meetings in Italy, Russia and the United States.

It singled out Damascus ally Russia for blame, saying its leaders were "ethically and politically responsible because they continue to support the (Syrian) regime with weapons."

The group had been due to attend a Friends of Syria meeting in Rome on Thursday, while Khatib had also been invited to Moscow, and to the United States.

"We cannot visit any country until there is a clear decision on this savage, aggressive regime," Khatib said of the government of Syrian President Bashar al-Assad.

Britain urged the opposition to reconsider its decision to pull out of the Rome, Washington and Moscow meetings, insisting "now is not the time to give up" on talks.

A Foreign Office spokeswoman said London was "preparing a further offer of support" for the National Coalition at the meeting in Rome.

But opposition spokesman Walid al-Bunni insisted that statements should be "accompanied by action" and said the world had a duty to protect Syrians from the regime of the "butcher" Assad.

"If you are our real friends, help us to stop the massacres that are being committed against our own people," Bunni said, challenging the United States to honour what he said were promises of support for democracy in Syria.

The opposition has repeatedly asked the international community to arm the rebels but Bunni fell short of repeating the demand, which Western powers have balked at, fearing the growing influence of radical Islamists.

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

A cabinet lineup and a premier would be chosen at a meeting on March 2 in Istanbul.

Analyst say an opposition-run government would fill a void in areas of Syria where the influence of Islamist rebels is growing.

Meanwhile, peace envoy Lakhdar Brahimi said a devastating attack on Thursday near the ruling Baath party's main offices in central Damascus had left about 100 people dead and another 250 wounded, described it as a "war crime."

Both the regime and opposition have blamed the attack on "terrorists."

In Aleppo, at least 37 people were killed and 150 wounded in missile strikes on the Tariq al-Bab district on Friday, said the Syrian Observatory for Human Rights.

Shelling of the city's Maadi district killing at least 14 civilians, among them 11 children, said the Britain-based watchdog, which gave a toll of at least 64 people killed in fighting across Syria on Saturday.

A military source said the army had sent reinforcements to Aleppo international airport, where fighting has raged between regime forces and rebels.

The rebels began an assault on several military and civilian airports in Aleppo province last week, in a bid to stop warplanes from taking off.

Elsewhere A Lebanese man was killed by gunfire on Saturday near a river that runs along Lebanon's northern border with Syria, a security source said.

The violence in Syria has increasingly spilled over into Lebanon, with cross-border shellings in the north and east.