The US military said Tuesday that Russia is preparing to deploy three naval ships to Syria for a mission that Moscow says is purely designed to ferry supplies to its base at the port of Tartus.

"We have no indication that these vessels and that material is being sent to Syria for any other purpose than that which the Russian military has acknowledged themselves, that it's for resupply and to help with force protection needs they have there at that facility," spokesman Captain John Kirby told a Pentagon news conference.

"Russian citizens have been threatened there in Syria, and their stated intention is that this is for force protection reasons of their own. That's what we believe to be the case," he said.

The three vessels were amphibious ships, said a US official, who spoke on condition of anonymity.

The three ships were in addition to a Russian cargo ship allegedly carrying attack helicopters to Syria, which turned back from British waters.

Britain's Foreign Secretary William Hague said Tuesday that the cargo vessel, the MV Alaed, had stopped off the Scottish coast after its British insurer said it had withdrawn cover because of the claims that it was carrying arms for President Bashar al-Assad's regime.

The United States has demanded Russia stop providing weapons or assistance to Damascus, a longtime ally of Moscow that offers naval bases with access to the eastern Mediterranean.

Russia has opposed US efforts at the UN Security Council to impose tough sanctions on Syria.

Any outside military support to the Assad regime that allows it to carry out attacks on the Syrian population "is intolerable and unacceptable," Kirby said.