New US Defence Secretary Chuck Hagel said the budget cuts due to go into effect later today will put the Pentagon's missions "at risk".
Speaking at his first Pentagon news conference, Hagel said, "This uncertainty puts at risk our ability to effectively fulfil all of our missions".
He also announced that the US Navy would gradually stand down four air wings and the Air Force would immediately cut flying hours.
However, Hagel said his department was determined to maintain the world's best fighting force.
More top news
Five people have been rescued from a small leisure boat on the Solent, after its engine failed and it began taking in water.
A US doctor has been accused of killing a lion during an illegal hunt, a week after it emerged dentist Walter Palmer killed Cecil the lion.
Mostly dry overnight in the east, strong winds and showery rain affecting Scotland & Northern Ireland