WASHINGTON -- Russia has sent advanced antiship cruise missiles to Syria, a move that illustrates the depth of its support for the Syrian government led by President Bashar al-Assad, American officials said Thursday.
Russia has previously provided a version of the missiles, called Yakhonts, to Syria. But those delivered recently are outfitted with an advanced radar that makes them more effective, according to American officials who are familiar with classified intelligence reports and would only discuss the shipment on the basis of anonymity.
Unlike Scud and other longer-range surface-to-surface missiles that the Assad government has used against opposition forces, the Yakhont antiship missile system provides the Syrian military a formidable weapon to counter any effort by international forces to reinforce Syrian opposition fighters by imposing a naval embargo, establishing a no-fly zone or carrying out limited airstrikes.
"It enables the regime to deter foreign forces looking to supply the opposition from the sea, or from undertaking a more active role if a no-fly zone or shipping embargo were to be declared at some point," said Nick Brown, editor in chief of IHS Jane's International Defense Review. "It's a real ship killer."
Jeffrey White, a fellow at the Washington Institute for Near East Policy and a former senior American intelligence official, said Syria's strengthened arsenal would "tend to push Western or allied naval activity further off the coast" and was also "a signal of the Russian commitment to the Syrian government."
The disclosure of the delivery comes as Russia and the United States are planning to convene an international conference that is aimed at ending the brutal conflict in Syria, which has killed more than 70,000. That conference is expected to be held in early June and to include representatives of the Assad government and the Syrian opposition.
Secretary of State John Kerry has repeatedly said that it is the United States' hope to change Mr. Assad's "calculations" about his ability to hold on to power so that he will allow negotiations for a political solution to the conflict. Mr. Kerry indicated that he had raised the issue of Russian arms deliveries to Syria during his recent visit to Moscow, but declined to provide details.
"I think we've made it crystal clear we would prefer that Russia was not supplying assistance," he said. "That hasn't changed."
American officials have been concerned that the flow of Russian and Iranian arms to Syria will buttress Mr. Assad's apparent belief that he can prevail militarily.
"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," Senator Bob Corker of Tennessee, the senior Republican on the Foreign Relations Committee, said in a statement on 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."
Senator Robert Menendez, Democrat of New Jersey and the committee chairman, added in a statement, "Russia is offering cover to a despotic ruler and defending a bankrupt regime."
Syria ordered the coastal defense version of the Yakhont system from Russia in 2007 and received the first batteries in early 2011, according to Jane's. The initial order covered 72 missiles, 36 launcher vehicles, and support equipment, and the systems have been displayed in the country.
The batteries are mobile, which makes them more difficult to attack. Each consists of missiles, a three-missile launcher and a command-and-control vehicle.
The missiles are about 22 feet long, carry either a high-explosive or armor-piercing warhead, and have a range of about 180 miles, according to Jane's.
They can be steered to a target's general location by longer-range radars, but each missile has its own radar to help evade a ship's defenses and home in as it approaches its target.
Two senior American officials said that the most recent shipment contained missiles with a more advanced guidance system than earlier shipments.
Russia has longstanding interests in Syria, including a naval base at the Mediterranean port of Tartus.
As the Syria crisis has escalated, Russia has gradually augmented its naval presence in the region. In January, more than two dozen Russian warships sailed to the Black and Mediterranean Seas to take part in what the Defense Ministry said was to be the country's largest naval exercise in decades, testing the ships' ability to deploy outside Russian waters.
A month later, after the Black Sea exercises ended, the Russian Defense Ministry news agency said that four large landing vessels were on their way to operations off the coast of Syria.
"Based on the results of the navy exercises in the Black and Mediterranean seas," the ministry said at the time, "the ministry leadership has taken a decision to continue combat duty by Russian warships in the Mediterranean."
Russia's diplomatic support of Syria has also bolstered the Assad government.
At the United Nations, the Russians recently blocked proposals that the Security Council mount a fact-finding trip to Jordan, Turkey and Lebanon to investigate the burgeoning flood of refugees, according to Western diplomats.
Jordan had sought the United Nations visit to make the point that the refugee situation was a threat to stability in the region, but Russia said that the trip was beyond the mandate of the Security Council, diplomats said.
When allegations that the Assad government had used chemical weapons surfaced, Russia also backed the Syrian government's refusal to allow the United Nations to carry out a wide-ranging investigation inside Syria -- which Russia's foreign minister, Sergey V. Lavrov, said was an attempt to "politicize the issue" and impose the "Iraqi scenario" on Syria.
Russian officials have repeatedly said that in selling arms to Syria, they are merely fulfilling old contracts. But some American officials worry that the deliveries are intended to limit the United States' options should it choose to intervene to help the rebels.
Russia, for example, previously shipped SA-17 surface-to-air missiles to Syria. Israel carried out an airstrike against trucks that were transporting the weapons near Damascus in January. Israel has not officially acknowledged the raid but has said it is prepared to intervene militarily to prevent any "game changing" weapons from being shipped to Hezbollah, the Lebanese militant group.
More recently, Israeli and American officials have urged Russia not to proceed with the sale of advanced S-300 air defense weapons. The Kremlin has yielded to American entreaties not to provide S-300s to Iran. But the denial of that sale, analysts say, has increased the pressure within Russia's military establishment to proceed with the delivery to Syria.
This article originally appeared in The New York Times.