North Korea has long sought an end to joint military exercises between US and South Korean forces, condemning them as a rehearsal for invasion, and conducting its own counter-drills and missile tests in response.

Tensions with Pyongyang inevitably escalate during the drills, which Washington and Seoul have long insisted are "defensive" in nature.

In a free-ranging press conference after his extraordinary summit with the young North Korean leader, Trump told reporters in Singapore he would end the "provocative" joint exercises, aping language that Pyongyang has long employed.

Seoul and Tokyo appeared to have been caught off-guard by the announcement.

Japanese defence minister Itsunori Onodera said Wednesday the drills were "vital" to regional security, and US Secretary of State Mike Pompeo insisted that suspension of the drills was contingent on productive negotiations in "good faith".

There is a precedent for stopping the drills. Joint exercises were halted in the 1990s during talks aimed at curbing Pyongyang's nuclear ambitions.

Here is a look at the main joint exercises:

- Key Resolve and Foal Eagle -

Key Resolve is a computer-simulated war game conducted by military commanders which usually begins in March and runs for about 10 days.

It always accompanies Foal Eagle, one of the world's largest field drills.

More than 200,000 South Korean and some 30,000 US soldiers -- including thousands flown in from abroad -- participate in the two-month Foal Eagle exercises.

This year's drills were delayed to avoid clashing with the Pyeongchang Winter Games in South Korea.

Pyongyang sent athletes, cheerleaders and the North Korean leader's sister Kim Yo Jong to the Olympics, starting a diplomatic thaw that culminated in the US-North Korea summit this week.

Foal Eagle was cut short afterwards by a month to maintain the rapprochement.

- Max Thunder -

Max Thunder is a massive air force drill that usually lasts two weeks.

This year, the two allies conducted Max Thunder in May, mobilising some 100 aircraft including advanced US F-22 fighter jets, which are feared by the North for their radar-avoiding and precision strike abilities.

Pyongyang strongly protested the drills, threatening to cancel its planned summit with the US and postponing a scheduled military meeting with Seoul.

- Ulchi Freedom Guardian -

This two-week drill usually starts in August and although it is largely a computerised command-and-control exercise, Pyongyang considers it a highly provocative rehearsal for invasion.

Following last year's drills, the North fired ballistic missiles over Japan, triggering global alarm and a furious response from Tokyo.