JAKARTA - The pilot of the missing plane had requested to fly at a higher altitude because of bad weather, but the request was turned down because there was another plane flying at that height.

Flight QZ8501 went missing early Sunday over the Java Sea between the islands of Belitung and Borneo, a heavily travelled shipping channel with shallow waters, according to Indonesian authorities, who are leading the search and locate operations.

Before the plane lost contact with air traffic controllers, one of the pilots had asked to fly at a higher altitude because of bad weather, officials said.

In a press conference following the flight's disappearance, the Transportation Ministry's acting director general for air transportation Djoko Murjatmodjo said that QZ8501 was initially flying at 32,000 feet.

However, the pilot contacted air traffic control in Jakarta at 6.12 am, requesting to fly at a higher altitude of 38,000 feet and to deviate to the left side of its route -- route M635 -- in an effort to avoid clouds.

"Air traffic control granted the request to deviate to the left side, but refused the request to fly higher because of air traffic," Mr Djoko was quoted as saying by Jakarta Post.

He told newspaper Kompas there was another plane flying at that height.

The tower lost contact with the plane shortly after that.