The opposition National Coalition announced late Friday it was pulling out of a Friends of Syria meeting in Rome and scheduled talks in Russia and the United States.
The statement said the move was to protest the lack of international condemnation of the "crimes committed against the Syrian people."
It singled out Russia in particular over Moscow's support for President Bashar al-Assad's regime.
"The international silence on the crimes committed every day against our people amounts to participating in two years of killings," the statement added.
"Hundreds of defenceless civilians are being killed by Scud missiles and the destruction of Aleppo, the city of history and civilisation, continues in a systematic manner," it added.
"In protest against this shameful international position, the coalition leadership has decided to suspend its participation in the Rome meeting of the Friends of Syria."
The meeting had been scheduled for an unspecified date next month.
"We hold the Russians leaders in particular ethically and politically responsible because they continue to support the (Damascus) regime with weapons," the National Coalition added.
The statement also said the opposition coalition was pulling out of scheduled meetings in Russia and the United States.