The United States and Germany on Friday warned Russia that arming President Bashar Assad's forces would hurt efforts to bring Syria's warring parties together for peace talks.

U.S. Secretary of State John Kerry, speaking after talks with his German counterpart, Guido Westerwelle, said Russian plans to send a sophisticated air defense system to Assad also put Israel's security at risk.

The United States and Russia are pressing for a peace conference in Geneva next month between Syria's government and opposition. Westerwelle called on Assad to "stop the violence and come to the negotiating table" and told Russia that sending the S-300 missiles to Assad "is totally wrong."

Earlier Friday, AFP quoted Russian newspapers as reporting that Russia is set to deliver a shipment of S-300 air defense missiles to Assad's regime in 2014, rejecting claims that the missiles are already in the hands of the Syrian government.

Assad said in a television interview broadcast Thursday that Moscow had fulfilled some of its weapons contracts recently, but was vague on whether this included the advanced S-300 air defense systems. In excerpts of the same interview released earlier Thursday, Assad said Syria had already received the missiles.

According to the Vedomosti newspaper, a source in the Russian defense industry has said it is unclear if the S-300s would reach Syria this year. The Kommersant daily, meanwhile, cited a source as saying that the missiles would reach the Assad regime in the second quarter of 2014.

Both of the sources quoted by the two dailies said that no missiles had been delivered so far. The contract for the S-300 shipment was finalized in 2010. Vedomosti said the contract is worth $1 billion, AFP said.

Additionally, Vedomosti's source said that although Moscow is publicly stating that the contract will be honored, and the shipment delivered, there is no guarantee that this will actually happen.

In another report, a Russian arms industry source told Interfax news agency on Friday that Moscow is unlikely to deliver the missiles to Syria before the autumn.