Russia will not supply new weapons to its Arab ally Syria while fighting there continues, Moscow arms export officials said Monday, while stressing that old contracts would be fulfilled.

"Russia like other countries is concerned about the situation in Syria," Russian agencies quoted Vyacheslav Dzirkaln, the deputy head of the Federal Service for Military Technical Cooperation, as saying.

"We are not talking about new deliveries of new weapons to this country," the Russian official said at the Farnborough Airshow. "Until the situation stabilises, no shipments of new weapons will be made."

Russia has long argued it has only been fulfilling existing contracts with Syria and is not supplying any of its most modern equipment under deals signed since the conflict began in March 2011.

Some of Russia's military enterprises are still conducting their own negotiations with Syria and strike deals that have, according to news reports, not been fulfilled up to this point.

In Washington, State Department spokesman Patrick Ventrell said: "Obviously, if it were true, it would be a good sign but we are still seeking further clarification from the Russians."

"We have repeatedly raised our concerns with the Russian government at a variety of levels. We have expressed our belief that continued arms sales to the regime will only further throw flames on the fire," he added.

UN spokesman Eduardo Del Buey said Secretary General Ban Ki-moon "has been very clear on the fact that all countries that have some influence on either side of the problem in Syria should exercise restraint in terms of arming."

Dzirkaln was specifically referring to a shipment of new Yak-130 trainer fighters that were on display at the British air show when making his comments to reporters.

Russian media said several of the Yak jets had been recently agreed for sale to President Bashar al-Assad's embattled Syrian regime. Dzirkaln said the fighters would not be delivered.

But the arms export agency's director, Alexander Fomin, also stressed that Moscow intended to continue supplying Assad with weapons and other military equipment agreed for sale under old deals.

"Syria is our old friend, and we fulfill all our obligations before our friends," Russian news agencies quoted Fomin as saying.

Russia defended last month's attempt to ship several repaired attack helicopters and an air defence system to Syria by saying that the contract for those had been signed some years ago.

The shipments were forced to return to Russia after its British insurer learned of the cargo's true content and withdrew cover.