A US Navy helicopter and a fighter jet crashed separately into the South China Sea within the space of an hour during routine operations conducted from the same aircraft carrier on Sunday, the US Navy said, adding all personnel were safe.

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

Speaking to reporters 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.

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

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 United States after the crashes, spokesperson Guo Jiakun told a press conference on Monday.

A Sea Hawk helicopter went down at about 2:45 pm 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 in a summit with Chinese President Xi Jinping on Thursday.