UK citizens are being warned against "all but essential" travel to Syria, after three weeks of unrest which have reportedly left 170 protesters dead.
The advice was issued in an update from the Foreign and Commonwealth Office.
Middle East and North Africa Minister Alistair Burt said that security in the country was "deteriorating".
Witnesses told the BBC that at least 23 protesters were killed in the southern city of Deraa on Friday, a month after anti-government protests first erupted.
The protests have posed an unprecedented challenge to President Bashar al-Assad's 11-year rule.
"The safety of British nationals is always our primary concern," Mr Burt said.
"We recommend that those in Syria exercise caution and maintain a high level of security awareness, particularly in public places and on the roads, and avoid large crowds and demonstrations."
Four people including a security forces officer are also said to have been shot dead in the Syrian port of Baniyas on Sunday.
The Foreign and Commonwealth Office advises against all travel to Baniyas and Deraa.
Foreign Secretary William Hague has branded the violence "unacceptable", adding: "Political reform is the only legitimate response to demands from the Syrian people."