SEOUL (REUTERS) - US President Donald Trump said on Saturday (Aug 10) that Kim Jong Un told him North Korea was ready to resume talks on its nuclear and missile programme as soon as US-South Korea military exercises ended.

North Korea fired what appeared to be two short-range missiles on Saturday, South Korea said, in a "show of force" against the military exercises.

More missile launches are highly probable, as the North Korean military is conducting its own summer drills, South Korea's Joint Chiefs of Staff said in a statement.

The launch came a few hours after Trump said he had received a "very beautiful letter" from Kim.

Trump tweeted on Saturday that Kim complained of the "ridiculous and expensive" exercises and would like to meet and start negotiations as soon as the joint drills were over.

He said Kim offered him "a small apology" for the flurry of missile tests, and that he assured him they would stop when the exercises end.

Trump said that Kim wants to meet once again to "start negotiations" after the drills conclude, and that he's looking "forward to seeing Kim Jong Un in the not too distant future!"

North Korea has fired a series of missiles and rockets since Kim and Trump agreed at a June 30 meeting to revive stalled denuclearisation talks.

A US official said that at least one projectile was launched and that it appeared to be similar to previous short-range missiles fired by Pyongyang.

Two missiles flew about 400km at a height of around 48km, according to the South Korean military.

Trump played down the recent North Korean weapons launches when he spoke to reporters earlier on Friday, saying: "I say it again: There have been no nuclear tests. The missile tests have all been short-range. No ballistic missile tests. No long-range missiles."

North Korea's state media has yet to confirm the launch, but in a commentary on Saturday it blamed the South for "building up arms against dialogue".

"All the facts prove that the South Korean authorities are hell-bent on arms buildup against their dialogue partner," the state-run KCNA news agency said.

"(South Korea is) the arch-criminal escalating tension in the Korean peninsula and the wrecker of its peace and stability."

South Korea called for Pyongyang to stop such launches.

The launch on Saturday morning was aimed at testing capabilities of North Korea's new type of short-range missile it is developing on its own, the South Korean presidential office said in a statement.

"Because of concerns that North Korea's series of launches can raise military tensions on the Korean Peninsula, ministers called for North Korea to stop it," the Blue House said, citing a meeting of South Korea's top security officials.

Kim has said the weapons tests were a response to US-South Korean military drills being held this month.

Trump has said Kim had written in his letter that he was "not happy" about the missile tests, which the North Korean leader has said were a response to US-South Korean military drills being held this month.

He said he could have another meeting with Kim.

The United States and South Korea have kicked off their largely computer-simulated exercises as an alternative to previous large-scale annual drills that were halted to expedite denuclearisation talks.

North Korea decries such exercises as a rehearsal for war aimed at toppling its leadership.

The projectiles were fired at dawn on Saturday from the area around the northeastern city of Hamhung, South Korea's Joint Chiefs of Staff said in a statement. Hamhung is known to have a solid-fuel rocket engine production site.

The projectiles were fired at dawn on Saturday from the area around the northeastern city of Hamhung, South Korea's Joint Chiefs of Staff said in a statement.

Large solid-fuel rocket engines for North Korea's ballistic missile programme are most likely being produced at a factory complex in Hamhung, monitoring group 38 North said last year.

Hamhung also has a testing site for those engines.

Kim Dong-yup, a former naval officer who teaches at Seoul's Kyungnam University, said the weapons tested on Saturday could be related to the completion of North Korea's new rocket artillery system that required multiple launches of the same kind.

Japan's ministry of defence said the projectiles did not pose an immediate security threat.