The two F-5E jets disappeared from radar around 3pm (0700 GMT) some 2.6 kilometres (1.6 miles) off the coast of rural southern Pingtung county, the National Rescue Command Centre said.

They were among four F-5E jets that took off some 30 minutes earlier for a routine training mission, the centre said, adding a helicopter and coastguard ships had joined the search.

Taiwan's aged fighter fleet has suffered a string of fatal accidents in recent years as the island's air force is kept under constant pressure by China.

Beijing views democratic and self-ruled Taiwan as its own territory and has vowed to one day seize it, by force if necessary.

Under President Xi Jinping, China has become markedly more hostile towards Taiwan and last year incursions by Beijing's fighter jets reached record highs.

The incursions force outgunned Taiwan to regularly scramble their own jets and keep pilots trained on a round-the-clock war footing that takes its toll on ageing aircraft frames and those flying them.

In October an F-5E pilot was killed when his plane crashed off the island's eastern coast.

A month later Taiwan temporarily grounded all F-16 fighters for safety checks after one went missing during a training exercise.

Last year, Chinese military jets made a record 380 incursions into Taiwan's defence zone, with some analysts warning that tensions between the two sides were at their highest since the mid-1990s.

The F-5E is an older generation fighter with a design that dates back to the 1960s.