US President Barack Obama on Thursday said he had "expressed some concerns" to France about its sale of warships to Russia.

"I have expressed some concerns, and I don't think I am alone," Obama said at a news conference in reply to a question about France's controversial decision to go ahead with a deal to sell two Mistral warships to Russia despite events in Ukraine.

"I think it would have been preferable to press the pause button," he added.

France has come under pressure from some European allies to pull the plug on the 1.2-billion-euro deal on the grounds Russia could use the Mistral-class helicopter carriers to threaten its Black Sea neighbours.

France agreed the sale under a 2011 contract, with the first vessel due in October and the second in 2015.

The Mistral, the second-largest ship in the French navy, is a high-seas military base that can transport up to 16 helicopters, four landing craft, 60 armed vehicles and some 700 troops.