US defense officials say the USS Nimitz aircraft carrier and four other ships in its strike group have moved into the Red Sea. They described the move on Monday as "prudent planning" in case the ships are needed for military action against Syria.

The nuclear-powered Nimitz is accompanied by a cruiser, and three destroyers. Officials added that the strike group had not received any orders to move into the Mediterranean, where five US destroyers and an amphibious ship remain poised for possible cruise missile strikes.

The US Navy doubled its presence in the eastern Mediterranean in the past week, effectively adding two destroyers to the three that generally patrol the region. The destroyers carry a combined load of about 200 Tomahawk missiles.

President Barack Obama backed off threats of an imminent strike until Congress had time to vote on its approval. Defense officials said the delay gave them more time to reassess which ships and what weapons will be kept in the region - and whether some may be allowed to leave.