Russia's shipment of anti-ship missiles to Syria will "embolden" the regime and fuel the country's civil war, the US military's top officer said Friday.

General Martin Dempsey's comments were the first official confirmation from the US government that Moscow had sent advanced "ship-killer" cruise missiles to President Bashar al-Assad's regime.

"It is at the very least an unfortunate decision that will embolden the regime and prolong the suffering," the chairman of the Joint Chiefs of Staff told a news conference when asked about the reported shipment of missiles.

"It's ill-timed and very unfortunate."

The New York Times first reported the arms shipment, quoting unnamed US officials, and that the missiles were outfitted with advanced radar.

Russia had previously delivered Yakhonts cruise missiles to Syria but those did not have the sophisticated radar, it said.

The anti-ship missiles could help Syria counter an attempt by outside powers to carry out air strikes from ships at sea or to impose a no-fly zone.

At the same press conference, Defense Secretary Chuck Hagel said the United States would continue to confer with Russia on the Syrian conflict and that both countries shared an interest in preventing a potential regional war.

He added that while diplomacy remained Washington's focus, the United States had not ruled out military action as an option.

The United States has long urged Russia to halt arms sales to Syria and has expressed particular concern about the planned delivery of sophisticated S-300 air defense weapons, which officials worry could complicate any international intervention and possibly fall into the hands of Lebanon's Hezbollah militants.

Asked about the missiles and US contingency plans, Dempsey said the SA-300 was a "more capable" system with a longer-range that would force any air attack to be carried out from a longer, "standoff" distance.

"It pushes the standoff distance a little more, increases risk but not impossible to overcome," the general said.

Apart from the technical capabilities of advanced missiles, such weapons could also cause the regime to overestimate its position and trigger a wider conflict.

"What I really worry about is that Assad will decide that since he's got these systems, he's somehow safer and more prone to a miscalculation," Dempsey said.

The general said the US military had no plans to block the delivery of the Russian-made weapons to Syria, saying "we do not have options to prevent the delivery of any military sales to the Syrians."

US Secretary of State John Kerry warned last week that such missiles sales would be "destabilizing."