Washington has condemned the Damascus regime for a missile strike that killed dozens of people, a day after Syria's opposition protested at the world's silence over the slaughter of civilians.
Fresh fighting broke out on Syria's border with Lebanon overnight, hours after shots fired from Syria killed a Lebanese man.
The United States condemned "in the strongest possible terms" the devastating missile strikes on Syria's second city Aleppo on Friday.
They were "the latest demonstrations of the Syrian regime's ruthlessness and its lack of compassion for the Syrian people it claims to represent", State Department spokeswoman Victoria Nuland said on Saturday.
At least 37 people were killed and 150 wounded in missile strikes on the Tariq al-Bab district on Friday, according to the Syrian Observatory for Human Rights.
Nuland repeated Washington's call for President Bashar al-Assad to step down.
"The Assad regime has no legitimacy and remains in power only through brute force," she said.
"The United States sees no indication that the brave Syrian people fighting against this aggression will accept these regime leaders, with the blood of so many Syrians on their hands, as part of a transition governing authority."
The comments from Washington came after Friday's statement from the Syrian National Coalition announcing a boycott of talks with world powers.
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", who were merely looking on as the Syrian people were being killed.
He 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.
Friday's statement singled out Syria's 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 next 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 Assad administration.
Britain urged the opposition on Saturday to reconsider, 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 said they wanted action, not just words. The world had a duty to protect Syrians from the regime of the "butcher" Assad, he added.
"If you are our real friends, help us to stop the massacres that are being committed against our own people," Bunni said.
Although the opposition has repeatedly asked the international community to arm the rebels, Western powers are reluctant, fearing the growing influence of radical Islamists.
On Friday, Bunni also 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 will be chosen at a meeting on March 2 in Istanbul.
Syrian troops clashed with fighters on the Lebanese side of the border overnight Saturday to Sunday, a senior Lebanese security official said.
Artillery, mortar fire and automatic weapons were used in the battle, the official said, but he had no information on casualties.
The violence in Syria has increasingly spilled over into Lebanon, with cross-border shelling in the north and east.