Syria has barred several ambassadors, including from Britain, the US and France, saying they were "no longer welcome" as the diplomatic crisis deepens.
The ambassadors had already been withdrawn, but the move was symbolic of how far the ties with Damascus have disintegrated during the 15-month rebel uprising.
Following last week's massacre in Houla, in which more than 100 people - including many women and children - were massacred, Western nations expelled Syrian diplomats in a co-ordinated action.
Syrian foreign ministry spokesman Jihad Makdessi said Damascus had decided to take a "reciprocal measure" against ambassadors from the US, Britain, Turkey, Switzerland, France, Italy and Spain.
A number of French, German, Canadian, Bulgarian and Belgian diplomats are also affected, Mr Makdessi said.
The United Nations has said pro-Assad regime gunmen were believed to be responsible for at least some of the killings in Houla.
But Syria's President Bashar al Assad has insisted his forces had nothing to do with the deaths and blamed them on "terrorists".
The regime is trying to crush the uprising against Mr Assad's rule, but the regime's deadly crackdown on the opposition has brought widespread international condemnation.
The UN has said at least one million Syrians were in urgent need of humanitarian aid, including people injured during fighting, and families who have lost jobs or homes.
Syria's government has agreed a deal with the UN and other international organisations to allow aid workers and supplies to enter four of the hardest-hit provinces - Daraa, Deir el Zour, Homs and Idlib.
UN spokesman John Ging said: "Whether this is a breakthrough or not will be measured in the coming weeks."
Meanwhile, Russian President Vladimir Putin and his Chinese counterpart Hu Jintao urged the international community to support UN envoy Kofi Annan's peace plan on Syria which includes a ceasefire.
Moscow and Beijing - both permanent members of the UN Security Council - have stifled efforts by Western powers to call for the removal of Mr Assad whose forces are thought to have killed more than 9,000 people in the uprising.
