Britain says it has summoned Syria's ambassador to warn new sanctions will target the regime's hierarchy if President Bashar Assad does not halt a violent crackdown on protests.

London's Foreign Office said on Friday that Syrian Ambassador Sami Khiyami had been called in for talks with political director Geoffrey Adams.

Germany on Wednesday summoned Syria's ambassador to Berlin to deliver a similarly stern message, as European nations consider extending travel bans and asset freezes the European Union imposed last week against 13 officials.

Western diplomats say up to 800 Syrians have been killed since demonstrations began in mid-March, and about 8,000 people detained.

Khiyami's invite to Britain's royal wedding was withdrawn over the regime's action.