A bomb dropped from a US F-18 fighter-bomber killed at least five people, including four US soldiers, in a training exercise in the Kuwaiti desert.
"The Americans missed their target during the air bombing exercise and killed probably six soldiers on the ground," an Arab defence source in Kuwait said.
One source said most of the dead were US soldiers training at the Udara range just south of the border with Kuwait's former occupier Iraq.
The US military along with Kuwait and other foreign forces based in the Gulf Arab state have set up a permanent training range at Udara, where almost year-round exercises code-named Intrinsic Action are held.
In Washington, the Pentagon officials said initial details of the accident were sketchy. They said the jet, based on the US aircraft carrier Harry Trumanin the Gulf, dropped at least one bomb that struck people on the ground.
"We lost some servicemen today in a training accident in Kuwait," President George W. Bush said in a speech during an appearance in Panama City, Florida. He asked the crowd for a moment of silence for the casualties.
The United States has maintained a military presence in Kuwait since the 1991 Gulf War which ended Iraq's seven-month occupation of its southern neighbour. Around 5,000 Americans are based in Kuwait.
Last month former US President George Bush visited Kuwait to mark the 10th anniversary of the liberation of the country by a U.S.-led coalition. He visited the Udara range and complemented the troops there on their professionalism.