The envoys of the US, UK, France and Turkey were among the diplomats designated "personae non gratae".
President Bashar al-Assad blamed "foreign meddling" for Syria's divisions in a speech on Sunday.
The move came as activists said at least seven people had been killed in violence across Syria on Tuesday.
Four civilians were killed overnight in a "huge military operation" in Kafrouaid, a village in the Jabal al-Zawiya area of the northern province of Idlib, the UK-based Syrian Observatory for Human Rights said.
Troops and pro-regime militiamen backed by tanks were also reported to have stormed the town of Kfar Zita in the central province of Hama, and killed two rebel fighters in the Mediterranean port city of Latakia.
Several villages south-west of the central city of Homs earlier came under intense army artillery- and mortar-fire, leaving three people dead, according to the Local Co-ordination Committees, an activist network.