Russia is sending the naval landing ship Nikolai Filchenkov to the eastern Mediterranean, says state news agency Interfax.
Russia, an ally of Syria's President Bashar al-Assad, says its vessels in the eastern Mediterranean guarantee security as the United States considers launching military strikes to punish Damascus for its alleged use of chemical weapons.
"The vessel will dock in Novorossiysk where it will take special cargo on board and head to the designated area of military service in the eastern Mediterranean," Interfax quoted an unnamed navy source as saying.
It gave no more details and Russia's defence ministry declined immediate comment.
Nikolai Filchenkov was not among vessels that the ministry said last month would enter the Mediterranean as part of a planned rotation.
Russia says it will not get involved militarily in Syria and opposes a possible US intervention, saying it would lack a mandate from the United Nations Security Council, where Moscow has blocked Western-led attempts to mount pressure on Assad.
The dispute over Syria has overshadowed a G20 summit in St Petersburg this week and there is little expectation that world powers will be able to overcome differences on the matter.
Meanwhile, the UN nuclear watchdog has received a request from Russia to assess the impact if a missile were to hit a small Syrian reactor and is considering the issue, the Vienna-based agency said on Friday.
Russia said this week a military strike on Syria could have catastrophic effects if the research reactor near Damascus that contains radioactive uranium was struck, "by design or by chance".
It called on the UN International Atomic Energy Agency (IAEA) to urgently assess the risk as the United States considers military action to punish Syria's government for an alleged gas attack.
"I can confirm that the IAEA has received a formal request from the Russian Federation. The agency is considering the questions raised," IAEA spokesperson Gill Tudor said in an email, giving no further detail.
Russia said nearby areas could be contaminated by highly enriched uranium and that it would be impossible to account for the nuclear material after such a strike, suggesting it could fall into the hands of people who might use it as a weapon.
Nuclear experts said the so-called Miniature Neutron Source Reactor (MNSR), a type of research reactor that is usually fuelled by highly enriched uranium, is small and that any radioactive fallout may pose a local hazard.
The amount that such a reactor usually holds, about 1kg of highly enriched uranium, is less than the 25kg that would be sufficient to build a bomb, said experts.
One Western diplomat in the Austrian capital played down the issue.
"It is very unlikely that something like this happens, and the quantity which is in this research reactor is very small," the envoy said.
"I have the feeling that the agency does not perceive this as a very grave concern."
Moscow has been the most powerful ally of al-Assad, shielding him from tougher UN resolutions and warning that any Western military attack on Syria would raise tension and undermine efforts to end the country's civil war.
In 2007, Israel bombed a desert site in Syria that US intelligence reports said was a nascent, North Korean-designed reactor geared to producing plutonium for nuclear weapons.
Syria said the site, at Deir al-Zor, was a conventional military facility. – Reuters