Russia's arms export agency said Wednesday its deliveries to Syria were in line with UN regulations but declined to comment specifically on US claims that it was sending attack helicopters to the regime.
Rosoboronexport "does not supply weapons and military technology in contradiction with UN Security Council requirements and other international agreements," a spokesman told the state-run RIA Novosti news agency.
The news report said the agency spokesman "did not comment" when asked about the specific charges.
US Secretary of State Hillary Clinton on Tuesday accused Russia of recently sending the shipment to its Soviet-era ally.
"We are concerned about the latest information we have that there are attack helicopters on the way from Russia to Syria, which will escalate the conflict quite dramatically," Clinton told a think-tank discussion in Washington.
The State Department failed to specify the source of Clinton's information while the Pentagon said it could not immediately confirm the claim.
But the comments prompted a call from permanent Security Council member France "for a complete halt to arms exports to the Syrian regime."
Russia has always argued that it was only supplying Syria with weapons such as air defence systems that could not be used against civilians in the army's 15-month stand-off with the armed opposition.
Several senior military analysts said Clinton may have been referring to helicopters that Moscow sold to Syria in the Soviet era and that were now undergoing repairs under contract in Russia.
They added that Moscow's last helicopter sales contract with Damascus expired about 20 years ago and was never renewed.
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"Military helicopters were supplied on a wide scale in Soviet times. But the last helicopter deliveries occurred in the early 1990s," Russia's Arms Exports magazine editor Andrei Frolov told RIA Novosti.
Clinton did not clarify whether the alleged shipments involved a new generation of helicopters that could not have been sold to Syria in the Soviet era.
Moscow Defence Brief editor Mikhail Barabanov said Clinton could only have be referring to "some Syrian Mi-24 or Mi-17 helicopters that were sent back to Russia for repairs."
London's International Institute for Strategic Studies estimates that Syria has more than 30 of the older Mi-24 models as well and about 80 Mi-8 and Mi-17 helicopters.
Russia came under fierce criticism from Western and Arab countries for vetoing two previous UN Security Council resolutions that would have sanctioned Assad for his use of force.
It took some steps in the past month to distance itself personally from Assad while also inviting more moderate opposition groups to Moscow for talks.
But it has insisted that it will continue supplying weapons to Syria under contracts that see Russia ship about $1 billion in weapons per year.
Russia's Deputy Foreign Minister Gennady Gatilov told reporters last month that Moscow believed "it would be wrong to leave the Syrian government without the means for self-defence."
Rosoboronexport is Russia's only arms export organisation and was recently contracted by the Pentagon to sell attack helicopters to the Afghan government.
That deal was heavily critcised by US Senators this week because of reports that Rosoboronexport was illicitly supplying Iran with rocket launch equipment.
The agency was also implicated in Russia's unconfirmed convert sea delivery to Syria of at least two shipment of munitions since the end of last year.