The international fallout continues over Syria after President Bashar al-Assad claimed a Russian contract to supply anti-aircraft missiles was being fulfilled. Russia says the arms have not yet reached Damascus.
The admission on a Hezbollah-owned TV channel on Thursday has further soured US-Russian relations.
Meeting with his German counterpart in Washington US Secretary of State, John Kerry said the delivery could prolong the conflict.
“It is not helpful to have the S-300 transferred to the region, while you are trying to organise this peace and create peace,” said Kerry. “It is not helpful to have a lot of other ammunition and other supplies overtly going on, not just from the Russians and they are supplying that kind of thing, but also from the Iranians and Hezbollah,” he continued.
German Foreign Minister Guido Westerwelle standing alongside Kerry urged Russia not to hinder peace efforts.
The US and Russia are attempting to bring together the Syrian government and opposition groups in Geneva in June to hammer out a peace deal. Though division amongst the opposition groups and Western powers organising the conference means no date has been set.
As diplomatic efforts stall, the battle for Qusair continues.
Many opposition fighters have arrived in the key town to counter government forces who, according to the Syrian Observatory for Human Rights, have re-taken much ground in the rebel-stronghold.
In other developments, the UN has classified al-Nusra, who fight on the side of the rebels, as a terrorist group due to their links with al-Qaeda.