The US has condemned recent rocket attacks on Syria's northern city of Aleppo, which opposition activists say killed dozens of people.
The US state department said the strikes were "the latest demonstrations of the Syrian regime's ruthlessness".
The statement came after Syria's opposition coalition pulled out of planned international talks because of the world's failure to stop violence.
The Syrian National Coalition (SNC) whose leader launched the diplomatic initiative three weeks ago said it had decided not to attend a meeting of the Friends of Syria Grouping in Rome next month.
It is also turning down invitations for talks in Washington and Moscow, the BBC reports.
A statement singled out Russia for supporting and supplying weapons to the Syrian ruler Bashar al-Assad.
SNC leader Moaz al-Khatib announced earlier this month that he was ready to meet Syrian government representatives to discuss an end to the violence.
The initiative was strongly backed by international powers including Russia and he was invited to Washington and Moscow. Now those visits will not take place.
The UN estimates more than 70,000 people have died since unrest against President Bashar al-Assad's rule began in March 2011.