A relative of the passengers of the flight QZ8501 waits for the latest news on the missing jetliner. (Associated Press)
The United States said on Sunday it was ready to help search for an AirAsia plane that disappeared while traveling from Indonesia to Singapore, but that so far it had not been asked.
The State Department said it was aware of the search and rescue operation under way being led by regional authorities, adding it "stands ready to assist in any way that's helpful."
The National Air Transportation Safety Board also said it was monitoring the situation and ready to aid the investigation if asked.
However, a Pentagon spokesman told AFP it was not "aware of any requests for assistance" so far.
The Airbus A320-200 disappeared en route from Surabaya in Indonesia's east Java to Singapore with 162 people on board, after the crew requested a change of flight path due to stormy weather.
The State Department confirmed that none of the passengers was traveling on a US passport.
The White House had said Saturday that President Barack Obama had been briefed on the plane's disappearance and was monitoring the situation.
The aircraft was operated by AirAsia Indonesia, a unit of Malaysian-based AirAsia which dominates Southeast Asia's booming low-cost airline market.
Earlier this year, a plane operated by another Malaysian airline disappeared with 239 passengers and crew over the Indian Ocean.
It still has not been located.
The United States has offered extensive assistance during the search for the missing jet, including sending aircraft to search for debris and a US Navy submersible Bluefin-21, which was used to scour the ocean floor.