Unnamed U.S. officials said information from Israel was being analyzed and refrained from commenting, reports in The Wall Street Journal and The New York Times said.
The reports say Syria is seeking to buy advanced Russian-made S300 missile batteries able to intercept manned aircraft and guided missiles.
White House spokesman Jay Carney told reporters aboard Air Force One en route to Texas the administration is aware of the reports and disappointed in Russia's continued support of the Assad regime.
"We have consistently called on Russia to cut off the Assad regime's supply of Russian weapons including air defense systems that are destabilizing to the region," Carney said. "We have also long said that Russia could play a more constructive role in Syria."
The Wall Street Journal said Syria has been making payments on a 2010 agreement with Russia to purchase four batteries for $900 million. The latest transaction took place this year, the newspaper said, adding the package included six launchers and 144 operational missiles each with a range of up to 200 miles. Delivery was expected in three months.
The sale, first reported in the Journal, came as U.S. Secretary of State John Kerry sought to enlist Russia's aid in facilitating a political transition in Syria and bringing an end to the conflict in which more than 70,000 people have been killed, The New York Times said.
Meanwhile, a leader of the Western-backed section of the Syria opposition complained, in an op-ed article Thursday in the Chicago Tribune, of a lack of action against the regime of Syria President Bashar Assad for the alleged use of chemical weapons
After indicating for months that chemical weapons use would change the Obama administration's view about intervening in Syria, "the administration's reluctance will only embolden the Assad regime, as well as other rogue governments that might possess chemical and biological weapons," wrote George Sabra, acting president of the Western-backed National Coalition for Syrian Revolution and Opposition Forces.
"U.S. inaction is giving the Assad regime, after two years of wanton bloodshed, a green light to take even more outrageous steps to kill innocents," Sabra said. "This undermines America's moral leadership."
Obama defended his cautious approach, saying Tuesday he would make decisions based on the facts and not "on a hope and a prayer."
Obama, who has said the use of chemical weapons in Syria would cross a "red line" and be a "game-changer," reiterated his view the evidence of such use was insufficient to require action or enable him to unite U.S. allies behind a response.
"The time has come for assertive U.S. action to lead international efforts to end the bloodshed," Sabra said in the newspaper, calling on Washington and its allies to promote his group, commonly called the Syrian National Coalition, "and an interim government, which are diligently implementing seeds of democracy while marginalizing the extremists."
He called on the international community to "urge the United Nations and Russia to send an unequivocal message to Assad to refrain from using chemical weapons and ballistic missiles against his people."
"The international community must also establish safe zones with protected air space from any shelling or missiles to allow for the free movement of Syrian refugees back into their country, and the delivery of humanitarian aid to internally displaced people. Also, it should provide medical facilities and supplies for injured civilians," Sabra said.
The Obama administration had no immediate response to the op-ed piece.
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