US Secretary of State John Kerry has warned Russia against its plan to provide Syria with long-range surface to air missiles, saying it would jeopardise any attempt to hold peace talks.
Mr Kerry made the comments in Washington DC on Friday after a meeting with German foreign minister Guido Westerwelle.
The ABC reports a preparatory meeting is to be held in Geneva next week on a joint US-Russian bid for talks to end Syria's two-year civil war.
"It is not helpful to have the S-300 (air defence missiles) transferred to the region while you are trying to organise this peace," he said.
"It is not helpful to have a lot of other ammunition and other supplies overtly going in not just from the Russians - and they are supplying that kind of thing - but also from the Iranians and Hezbollah."
Mr Kerry said the delivery of missiles would have a "profoundly negative impact" on the balance of interests and the stability of the region.
President Bashar al-Assad suggested this week that his government has already received the S-300 missiles.
Newspapers in Russia rejected the claim, saying Moscow may not deliver the missiles this year.
The Interfax news agency also reported that Moscow could also supply 10 ultra-modern MiG-29 fighter jets to Syria under a possible contract being discussed with Damascus.
Mr Westerwelle said that delivering the missiles "is totally wrong." He called on Mr Assad "to ... come to the negotiating table" and urged the opposition to unite and participate in the meeting.
On Friday, the European Union formally waived an arms embargo against the rebels, from 1 June.