North Korea fired three short-range ballistic missiles Saturday, the US military said, following weeks of heightened tensions between Washington and Pyongyang.

The missile launches come as tens of thousands of South Korean and US troops take part in joint military drills in the South, which Pyongyang views as a highly provocative rehearsal for an invasion of its own territory.

Two of the missiles failed in flight and the third blew up "almost immediately", said a spokesman for US Pacific Command.

None of the missiles, which were launched near Kittaeryong, had posed a threat to either North America or the US Pacific territory of Guam, the spokesman said.

"The first and third missiles... failed in flight. The second missile launch... appears to have blown up almost immediately," said the spokesman, Commander Dave Benham, adding that the launches happened over a span of 30 minutes.

White House press secretary Sarah Sanders said US President Donald Trump was aware of the launches.

"In regards to activity in North Korea tonight, the president has been briefed and we are monitoring the situation," she said.

The launches came as North Korea state media reported that leader Kim Jong-Un oversaw a military exercise simulating a special forces assault on South Korean border islands involving aircraft, "multiple-missile launchers" and howitzers.

Shells hit islands standing in for South Korea's Paekryong and Taeyonphyong islands while special forces landed in rubber boats or parachuted in and "wiped out the desperate enemy with various combat methods," the Korean Central News Agency said.

"Kim Jong-Un expressed great satisfaction over the successful target-striking contest," it said.

Advances in missile technology

South Korea's defence ministry said "unidentified projectiles", fired at 6:49 am (2149 GMT Friday), flew some 250 kilometres (155 miles) towards the Sea of Japan.

The North's move was immediately reported to South Korean President Moon Jae-in.

"The military is keeping a tight surveillance over the North to cope with further provocations", the ministry added.

Yoshihide Suga, the Japanese government's top spokesman, told reporters in Tokyo Saturday morning: "We confirmed that no ballistic missiles have fallen onto our country's territory or EEZ (exclusive economic zone)."

"We confirmed there was no direct impact on our country's security. Our prime minister told us to remain on high alert and do our best to respond to any situations in order to protect our people's lives and property."

The North tends to carry out test-firing of missiles or other projectiles -- including those from long-range multiple rocket launchers -- in response to the US-South Korea joint military exercises.

The "Ulchi Freedom Guardian" war games are a largely computer-simulated exercise that runs for two weeks in the South. Pyongyang threatens strong military counteraction each year.

In recent weeks the North has threatened to fire a salvo of missiles toward the US Pacific territory of Guam, but has since backed away from the plan and tensions have eased.

Under Kim, Pyongyang has made rapid strides in its ballistic missile technology, which it is banned from pursuing under United Nations resolutions that have slapped it with seven sets of sanctions.

Last month it carried out two successful ICBM launches, overseen by Kim and apparently bringing most of its sworn enemy the United States into range for the first time.