WASHINGTON (Reuters) - The United States opposes France's plan to sell Mistral helicopter carriers to Russia, a senior U.S. administration official told reporters on Monday, echoing concerns expressed on Monday by British Prime Minister David Cameron.

President Barack Obama expressed concerns about the French contract in June because of Russia's support for separatists in eastern Ukraine.

Mr Cameron on Monday said the sale would be "unthinkable" in Britain after a passenger jet was shot down last week from eastern Ukraine.