The US Navy has deployed an amphibious transport ship to the Mediterranean, where five destroyers are already in place for possible missile strikes on Syria, a defense official said on Sunday.
The USS San Antonio, with several helicopters and hundreds of Marines on board, is "on station in the Eastern Mediterranean" but "has received no specific tasking," said the defense official, who spoke on condition of anonymity.
Unlike the destroyers deployed to the area, the San Antonio carries no Tomahawk cruise missiles but can ferry up to four helicopters and is designed to bring Marines ashore by chopper or landing craft.
No amphibious landing is in the works, however, as President Barack Obama has ruled out any "boots on the ground" if the US takes military action against the Syrian regime.
There are five destroyers off the coast of Syria, the USS Stout, Mahan, Ramage, Barry and Graveley, that are ready to fire cruise missiles if Obama gives the order.
In a surprise move, Obama on Saturday delayed threatened missile strikes against Damascus that had appeared imminent, saying he first would seek formal approval from Congress.
His decision puts off any military action until at least September 9, when US lawmakers reconvene after their summer break.
Some analysts said Syrian President Bashar al-Assad would benefit from the delay, as it would give his regime more time to disperse military hardware that would targeted in any missile strikes.
But another defense official insisted the delay would not undercut the effect of US military action, if and when Obama issues the order to go ahead.
"Our intelligence and targeting capabilities offer the president and the nation tremendous advantages," said the official, who asked not to be named.
"While providing advance warning to the Syrian government of a US strike may add some complexities, it does not translate into protection to their military resources," the official told AFP.
The American military will "continue to update our targeting options," he added.