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.”