Any attempt by Britain to arm the Syrian rebels would be a breach of international law, Russia has warned.
Following talks in London with UK foreign and defence ministers, Russian foreign minister Sergei Lavrov made clear Moscow would oppose any move to supply weapons to the opposition fighting President Bashar Assad.
"In our point of view it is a violation of international law," he told a joint news conference at Lancaster House.
However Foreign Secretary William Hague and Defence Secretary Philip Hammond made clear that nothing was ruled out in terms of arming the rebels.
"We have never ruled out anything in the future," Mr Hague said, adding it was not clear how grave the situation would become.
"Anything we do will be legal and clearly stated to our country and to the international community."