A US Navy helicopter and a fighter jet have crashed separately into the South China Sea within the space of an hour during routine operations conducted from the same aircraft carrier.

The incidents on Sunday came while US President Donald Trump was on a visit to Asia.

Speaking aboard Air Force One on Monday, Trump said the crashes were unusual and could be due to “bad fuel”, adding that what caused them would likely soon be known.

“All personnel involved are safe and in stable condition. The cause of both incidents is currently under investigation,” the US Pacific Fleet said in a post on X.

It did not disclose the location of the aircraft carrier USS Nimitz, from which both the helicopter and jet were conducting routine operations.

China’s foreign ministry offered humanitarian assistance to the US after the crashes, spokesperson Guo Jiakun said on Monday.

A Sea Hawk helicopter went down about 2.45pm on Sunday and shortly afterward, an F/A-18F Super Hornet fighter jet also crashed in the South China Sea, the navy said.

Trump is heading to Tokyo and will conclude his trip at a summit with Chinese President Xi Jinping on Thursday.

-AAP