Any attempt by Britain to arm the Syrian rebels would be a breach of international law, Russia warned today.
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 wasn't clear how grave the situation would become.
"Anything we do will be legal and clearly stated to our country and to the international community."
Mr Lavrov's comments come after British ministers have talked publicly about arming the rebels if the situation in Syria continues to deteriorate.
Last week Mr Hague announced that the UK was stepping up "non-lethal" assistance to the opposition including armoured cars, body armour and secure communications equipment.
However Mr Lavrov warned that the "most dangerous and effective" group fighting the Assad regime was the Islamist Jabhat al Nusra which has been declared a terrorist organisation by the United States.
"We should think about what is going to happen next and have a broader understanding," he said.
The talks were intended to help rebuild UK-Russia relations which have been at a low ebb since the murder of the former KGB agent Alexander Litvinenko in London in 2006.
Despite their continuing differences - including on the need for a new United Nations Security Council resolution on Syria - both Mr Hague and Mr Lavrov stressed the importance of the two countries continuing to work together.
"This meeting is an important milestone in strengthening these working relationships," Mr Hague said.
"It is entirely well worth continuing our efforts in the coming weeks to bridge these differences. It is vital to do so. It is vital that there is a political process and transition in Syria."
Mr Hammond disclosed that Britain was offering security assistance to the Russians in relation to the Winter Olympics next year at Sochi based on the experience of the 2012 London Games.