Foreign Minister Sergei Lavrov, speaking Friday at a news conference with U.N. Secretary-General Ban Ki-moon, accused the media of sensationalizing the delivery of the newly advanced weapons, RIA Novosti reported.
"I don't understand why the mass media are trying to make a sensation of it," Lavrov was quoted as saying by ITAR-Tass. "We haven't concealed that we supply weapons to Syria under signed contracts without violating any international treaties or our legislation."
U.S. officials, however, say the delivery of Yakhont, or the P-800 Oniks, missiles demonstrates the depth of Moscow's support for the regime of Syrian President Bashar Assad, The New York Times reported.
Other U.S. officials told The Wall Street Journal the missiles were most likely intended for delivery to the Hezbollah militant group in Lebanon.
Israeli and Western intelligence services say the missiles could be transferred to Hezbollah within days, the Journal said.
U.S. officials told the Times the missiles are a new version of the Yakhont missiles Russia supplied to Syria in recent years. The new ones are outfitted with advanced radar that makes them more effective, the officials said.
The 3.5-ton missiles can go twice the speed of sound, carry either a high-explosive or armor-piercing warhead and have a range of about 180 miles.
Russian officials have repeatedly said Moscow was simply fulfilling old contracts in selling arms to Syria.
Israeli warplanes carried out airstrikes in Syria May 3 and 5. The first, at Damascus International Airport, is reported to have sought to destroy weapons being sent to Hezbollah from Iran. The second is reported to have hit Syrian Republican Guard bases and long-range-missile storehouses, along with a military research center U.S. officials say was the country's main chemical weapons site.
Jerusalem did not confirm either attack, which followed another one Jan. 31 that was reported to have hit a convoy carrying advanced antiaircraft weapons to the Hezbollah militia.
Moscow and Damascus strongly protested the Israeli strikes, with Syrian Deputy Foreign Minister Faisal Mekdad vowing Damascus would "respond immediately and harshly to any additional attack by Israel."
A senior Israeli official said Wednesday the Assad regime faced dire consequences if it countered any future Israeli military strikes on Syria to stop arms transfers.
"If Syrian President Assad reacts by attacking Israel, or tries to strike Israel through his terrorist proxies, he will risk forfeiting his regime, for Israel will retaliate," the unnamed official told The New York Times in Washington.
Some U.S. officials told the Times they were concerned the missile deliveries were intended to limit Washington's options if it chooses to intervene to help the rebels.
The weapons could counter any effort to impose a naval embargo, establish a no-fly zone or carry out limited airstrikes, the officials told the newspaper.
"This weapons transfer is obviously disappointing and will set back efforts to promote the political transition that is in the best interests of the Syrian people and the region," Senate Foreign Relations Committee member Bob Corker, R-Tenn., said in a statement Thursday night.
"There is now greater urgency for the U.S. to step up assistance to the moderate opposition forces who can lead Syria after Assad," he said.
At the same time Russian warships patrolling the eastern Mediterranean Sea near Moscow's Syrian naval base are seen by U.S. and European officials as a warning to the West and Israel to stay out of Syria's bloody civil war, the Journal said.
"They are very much interested in letting people know they are a blue-water navy," a U.S. official told the Journal.
The U.S. Navy has two destroyers in the eastern Mediterranean and an aircraft carrier battle group in the Persian Gulf.
The disclosures of the missile delivery and the naval actions come days after Moscow and Washington worked publicly to put together an international conference aimed at ending Syria's civil war, which has killed more than 70,000 people.
That conference, likely to be held in early June, is expected to include Assad regime and Syrian opposition representatives.
The meeting is not expected to lead to a Syrian political transition, with tensions heightened in the region and Washington and Moscow backing opposing groups.