North Korea fired two ballistic missiles early today South Korea's military has said - the nuclear-armed country's fourth such launch this week, as Seoul, Tokyo and Washington ramp up joint military drills to counter Pyongyang.

South Korea, Japan and the US staged anti-submarine drills yesterday - the first in five years - just days after Washington and Seoul's navies conducted large-scale exercises in waters off the peninsula.

US Vice President Kamala Harris was also in Seoul on Thursday and toured the heavily fortified Demilitarised Zone which divides the peninsula, on a trip which aimed to underscore the "ironclad" US commitment to South Korea's defence against the North.

With talks long stalled, Pyongyang has doubled down on its banned weapons programmes, conducting a record-breaking blitz of tests this year and revising its laws to declare itself an "irreversible" nuclear power.

South Korea's military said it had "detected two short-range missiles between 6.45am and 7.03am local time fired from the Sunan area in Pyongyang into the East Sea", it said, referring to the body of water also known as the Sea of Japan.

"The military is maintaining the utmost readiness in close coordination with the United States," Seoul's Joint Chiefs of Staff added in a statement.

Japan also confirmed the launch of two apparent ballistic missiles, saying they appeared to have landed outside Japan's exclusive economic zones.

Toshiro Ino, Japan's vice defence minister, said the missiles "appear to have flown in irregular trajectories".

"North Korea has been repeating missile launches at an unprecedented pace," he said.

Experts say the irregular trajectories indicate the missiles are capable of manoeuvering in flight, making them harder to track and intercept.

US Vice President Kamala Harris in the DMZ in Paju, South Korea, on Thursday

Harris trip

North Korea marked Ms Harris's trip to Seoul with a flurry of missile launches - firing off short-range ballistic missiles on Sunday, Wednesday and Thursday, including just hours after the vice president flew out of South Korea.

Washington has about 28,500 troops stationed in South Korea to help protect it from the North.

Under Seoul's new President Yoon Suk-yeol, who took office in May, the two countries have boosted joint exercises, which they insist are purely defensive.

Just before Ms Harris's arrival in Seoul, Washington sent the nuclear-powered USS Ronald Reagan aircraft carrier to South Korea to conduct a large-scale joint naval exercise, in a show of force against Pyongyang.

Such drills infuriate North Korea, which sees them as rehearsals for an invasion.

"North Korea's short-range ballistic tests are less important than a nuclear test but still violate UN Security Council resolutions," said Leif-Eric Easley, a professor at Ewha University in Seoul, adding that the timing was "provocative".

North Korea is "rapidly modernising weapons and taking advantage of a world divided by US-China rivalry and Russia's annexation of more Ukrainian territory", he said.

"Pyongyang's actions again make clear the need for Washington and Seoul to reinforce military deterrence, tighten economic sanctions, and increase policy coordination with Tokyo," he added.

South Korean and US officials have also been warning for months that North Korean leader Kim Jong-un is preparing to conduct another nuclear test.

On Wednesday, the South's spy agency said North Korea's next nuclear test could happen in the window between China's upcoming party congress on 16 October and the US midterm elections on 7 November.

North Korea, which is under multiple UN sanctions for its weapons programmes, typically seeks to maximise the geopolitical impact of its tests with careful timing.

The isolated regime has tested nuclear weapons six times since 2006, most recently in 2017.