HANOI - A Russian-made Vietnamese fighter jet was missing on Tuesday after disappearing off the radar during a training flight along the central part of the country's South China Sea coastline, a military official said.

The Sukhoi SU-30 MK2 was flying 30-40 km off the coast of Nghe An province next to China's Hainan island when it went missing and practice flights by the same jet model had since been suspended, said Senior Lieutenant General Vo Van Tuan, the deputy armed forces chief of staff.

"We haven't found the jet yet, we are still looking," he told Reuters.

Asked about the possible reason for its disappearance, Tuan said: "We have to find it to know it."

The incident follows a series of mishaps in the past two years with its ageing helicopters and at a time when Vietnam recalibrates its defense strategy and oversees its biggest military buildup in four decades.

The country is seeking to modernize its defense capability, ostensibly as a deterrent against the military rise of Beijing and its growing assertiveness in the South China Sea, over which the two communist neighbors have competing claims.

The missing SU-30 MK2 fighter-bomber was one of 30 in use by Vietnam. Fighter jets are on its shopping list and its options are expected to expand following last month's lifting of a U.S. lethal arms embargo on a former enemy Washington is eager to turn into its newest Asian ally.

Vietnam has been in talks with Western and U.S. arms manufacturers towards boosting its air force with jets, helicopters and maritime patrol aircraft, although experts say it is likely Russia will continue to be its main supplier.

China claims most of the energy-rich South China Sea through which about $5 trillion in ship-borne trade passes every year. Brunei, Malaysia, the Philippines, Taiwan and Vietnam also have claims.