Syria's opposition National Coalition said it was pulling out of several international meetings to protest at the "international silence" despite the slaughter of civilians in the conflict.
The announcement came after the coalition had said it would form a government to run "liberated areas" of Syria, and as international condemnation mounted against Thursday's devastating attacks in Damascus that left around 100 people dead.
In further violence Friday, monitors said more than 12 people had been killed when buildings collapsed after a missile strike on the city of Aleppo.
The National Coalition said it was pulling out of meetings in Italy, Russia and the United States, to protest against the "shameful" lack of international condemnation of "crimes committed against the Syrian people".
The group had been due to attend a Friends of Syria meeting in Rome next Thursday where US Secretary of State John Kerry is also expected. National 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," said the statement.
"We hold the Russian leaders in particular ethically and politically responsible because they continue to support the (Damascus) regime with weapons," the National Coalition added.
Already Friday, coalition spokesman Walid al-Bunni had announced plans for a government for "liberated areas" following a meeting in Cairo.
They would decide on its composition and choose its leader at a meeting on March 2, he added.