Vietnam launched a search mission on Thursday for two fighter pilots who ejected over the South China Sea during a training exercise involving two Russian-built fighter jets, a provincial official said.

The pilots were flying separately in two Sukhoi Su-22 jets but lost contact off the coast of south-central Binh Thuan province around noon local time, the Defense Ministry said.

The incident is the latest in a series of setbacks for Vietnam's military, which has seen two fatal helicopter accidents in the past nine months, killing a combined 24 personnel.

The Defense Ministry said in a statement that search teams had located four reserve fuel tanks and it was trying to determine what happened. It did not say whether or not the two planes had crashed.

Nguyen Hung Tan, chief administrator at the provincial committee for floods and storms prevention, said the pilots had ejected and their whereabouts were unknown.

"The two pilots jumped out of the planes and parachuted," Tan told Reuters.