More shots have been fired in the diplomatic row over who is supplying weapons to Syria.
Britain says it has intervened to stop a vessel carrying Russian-made attack helicopters from passing off the coast of Scotland en-route to Syria.
Moscow has faced increasing Western criticism over arms supplies to Syria, where the United Nations says government forces have killed more than 10,000 people in a crackdown.
Last month, the U.S. described the delivery of heavy Russian weapons in a shipment as “reprehensible”.
On the ground in Syria, the government has blamed rebel fighters for obstructing efforts to get people out to safety.
Activists say the army has intensified shelling of residential districts of Homs, and the head of the UN monitors says he is increasingly worried about the fate of civilians trapped there.