Two Taiwanese fighter jets crashed on Monday in the third such incident in the past half year, at a time when the Beijing-claimed island's armed forced are under increasing pressure to intercept Chinese aircraft on an almost daily basis.

While Taiwan's air force is well trained and well equipped, mostly with US-made equipment, it is dwarfed by China's. Beijing views the democratic island as its own territory and has never renounced the use of force to bring it under Chinese control.

Taiwan's National Rescue Command Centre said two air force F-5E fighters, each with one pilot aboard, crashed into the sea off the island's southeastern coast after they apparently collided in mid-air during a training mission.

An air force helicopter, coast guard and other rescue ships have been scrambled to look for the pilots, it added.

Taiwan's Defence Ministry said it was working on a statement, and provided no other immediate comment. The official Central News Agency said the air force had now grounded the F-5 fleet that operates from the Chihhang air base, where the aircraft are based.

The U.S.-built F-5 fighters first entered service in Taiwan in the late 1970s and have been mostly been retired from front-line activities, though some are still used for training and as a back-up for the main fleet.

Another F-5 crashed in October, killing the pilot. The following month a much more modern F-16 crashed off Taiwan's east coast, whose pilot also died.

In January of last year, Taiwan's top military official was among eight people killed after a helicopter carrying them to visit soldiers crashed in a mountainous area near the capital Taipei.

The incidents have raised concern about both training and maintenance, but also the pressure the air force is under to respond to repeated Chinese flights near the island.

Taiwan's Defence Ministry has warned Chinese aircraft, including drones, are flying repeatedly in Taiwan's air defence identification zone, seeking to wear out Taiwan's air force.