The Britain-based Syrian Observatory for Human Rights has said "tens of people" were killed in an air strike by a Syrian Air Force fighter jet, which hit a bakery in a town near the central city of Hama.
Rebels have been clashing with government forces is recent days in the region around Halfaya, some 25 kms (15 miles) north west of Hama.
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Among those murdered was Keith Palmer, an unarmed constable who "did his job and never wanted any fuss."
Top temperature 14 Celsius (57 F).
The Prime Minister will say Moscow's brazen flouting of international law represents a threat to the basis of democracy across Europe.