US warns of toothpaste bombs on flights

The US government has warned American and foreign airlines that terrorists could try to place explosives disguised as toothpaste on Russia-bound flights.

An official says it has information "specifically targeting flights to Russia," where the Winter Olympics will begin in the resort city of Sochi on Friday.

In a statement, the Department of Homeland Security (DHS) said that, "out of an abundance of caution," it regularly shares relevant information with partners both at home and abroad.

"While we are not aware of a specific threat to the homeland at this time, this routine communication is an important part of our commitment to making sure we meet that priority," it said.

"As always, our security apparatus includes a number of measures, both seen and unseen, and DHS will continue to adjust security measures to fit an ever evolving threat environment," it said.

Security at the February 7-23 Games has been a major concern for Washington following two deadly December suicide attacks in the southern Russian city of Volgograd.

Adding to the concern is a stream of threats from Islamist militants in the volatile northern Caucasus region.

Earlier on Wednesday, officials said two US warships had arrived in the Black Sea and would stand ready to offer assistance in the case of a security emergency at the massive sports event.