Vietnam has sent four aircraft and dozens of vessels to search for a jet fighter with two pilots that went missing on a training flight.

The last contact with the Russian-made Sukhoi Su-30 MK2 jet fighter was Tuesday morning.

The state-run Tuoi Tre newspaper quoted fisherman Le Van Cuong as saying he saw a jet fighter crash near Mat island off the coast of central Nghe An province.

Vietnam has reported several crashes of military helicopters in recent years.

Faced with China's increased assertiveness over rival territorial claims in the South China Sea, Vietnam has stepped up the modernization of its military with the purchase of six Russian-made submarines and a number of Russian fighter planes. It is also looking to acquire patrol boats from Japan and India.