The United States and Germany on Friday warned Russia not to endanger a planned peace conference for Syria or alter the balance of power in the Middle East by providing an advanced air defence system to President Bashar al-Assad's embattled regime.

U.S. Secretary of State John Kerry and German Foreign Minister Guido Westerwelle said the transfer of S-300 missiles from Russia to Syria would prolong the country's civil war, imperil attempts to form a transitional government through negotiation and hurt Israel's strategic interests.

"It is not helpful to have the S-300 transferred to the region while we are trying to organize this peace [conference] and create peace," Kerry told reporters at a joint news conference with Westerwelle after they met at the U.S. State Department.

"We ask them again not to upset the balance within the region with respect to Israel," he said. "The weaponry that is being provided to Assad, whether it is an old contract or not, has a profoundly negative impact on the balance of interests and the stability of the region and it does put Israel at risk. It is not in our judgment responsible because of the size of the weapons, the nature of the weapons and what it does to the region in terms of Israel's security, so we hope that they will refrain from that in the interests of making this peace conference work."

Westerwelle called on Russia not to "spoil" the conference, which was planned for Geneva next month but has been delayed until July at the earliest.

U.S. testing claims of chemical weapon use

Kerry and Westerwelle spoke a day after Assad suggested that his regime received a first shipment of the state-of-the-art anti-aircraft system, which would reduce pressure on it to negotiate with the opposition, make foreign intervention more difficult and alarm Israel. The United States has made Israel's qualitative military edge one of its prime strategic interests in the region.

Russian officials have said they will supply the S-300s to Syria under an existing contract. They have also said that they will try to convince the Assad regime to participate in the Geneva peace talks. But the Syrian opposition has said it will not attend the conference while massacres are continuing.

Kerry said he believed the opposition would attend the meeting once it straightens out questions about its leadership.

He also warned again that the use of chemical weapons is a "red line" for the Obama administration. At the same time, he said intelligence and evidence that such weapons have, in fact, been used, is still being vetted for 100 per cent certainty.

"We are doing our due diligence, as we should do given past experience, to make certain that the intelligence is correct, that the evidence is real and then make judgments that are appropriate," Kerry said.

Russian arms dealer plans to send Syria new jets

Russia's MiG aircraft maker, meanwhile, said it plans to sign a agreement to ship new fighter jets to Syria.

Sergei Korotkov, general director of the MiG company, says a Syrian delegation was in Moscow to discuss a contract for "more than 10" MiG 29 jets. (Dima Korotayev/Reuters)

MiG's director general, Sergei Korotkov, said a Syrian delegation was in Moscow to discuss the details of a new contract for the delivery of MiG-29 M/M2 fighters.

In remarks carried by Russian news agencies on Friday, he said Syria wants to buy "more than 10" such fighters, but wouldn't give the exact number.

The significance of his comments was unclear. A MiG spokesman wouldn't comment on Korotkov's statement, and the MiG chief could be referring to a deal the company previously negotiated with Syria that apparently has been put on hold amid Syria's brutal two-year civil war.

More than 70,000 people have died in the fighting and millions of Syrians have fled the country.

Moscow has shipped billions of dollars' worth of missiles, combat jets, tanks, artillery and other military gear to Syria over more than four decades. Syria now is Russia's last remaining ally in the Middle East and hosts the only naval base Moscow has outside the former Soviet Union.

Russia has shielded Assad from UN sanctions and has continued to provide his regime with weapons despite the uprising that began in March 2011.

Kremlin might think twice about jet delivery

Russian media reports say Syria placed an order a few years ago for 12 MiG-29 M2 fighters with an option of buying another 12. The Stockholm Peace Research Institute also has reported that Russia planned to provide Syria with 24 of the aircraft.

The MiG-29 M2 is an advanced version of the MiG-29 twin-engine fighter jet, which has been a mainstay of the Soviet and Russian air force since mid-1980s. Syria had about 20 fighters of the original make among scores of other Soviet- and Russian-built aircraft.

Russia has said it's only providing Assad with weapons intended to protect Syria from a foreign invasion, such as air defence missile systems, and is not delivering weapons that could be used in the civil war.

But the delivery of MiGs would contradict that claim and expose Russia to global criticism, so the Kremlin might think twice before giving the go-ahead.

Another recent Russian jet deal with Damascus, a contract to deliver Yak-130 combat training jets that could also be used for ground attacks, apparently has been put on hold amid the civil war.