North Korea launched two ballistic missiles early Saturday, the South Korean military said, the nuclear-armed country’s fourth such launch this week, as Seoul, Tokyo and Washington step up joint military exercises to counter Pyongyang.

South Korea, Japan and the US on Friday held anti-submarine drills – the first in five years – just days after the Washington and Seoul navies conducted large-scale drills in waters off the peninsula.

US Vice President Kamala Harris was in Seoul on Thursday touring the heavily fortified demilitarized zone dividing the peninsula in a trip aimed at underscoring her country’s “iron-clad” commitment to defending South Korea against the North.

As talks have long stalled, Pyongyang has doubled down on its banned weapons programs, conducted a record-breaking blitz of tests this year and revised its laws to declare itself an “irreversible” nuclear power.

The South Korean military said it “detected two short-range missiles launched into the East Sea from the Sunan area of ??Pyongyang between 0645 and 0703,” referring to the body of water also known as the Sea of ??Japan.

The missiles “flew approximately 350 km (217 miles) at an altitude of 30 km at a speed of Mach 6,” the Seoul chiefs of staff said in a statement, calling the launches “a serious provocation.”

Tokyo also confirmed the launch, saying the missiles landed outside of Japan’s exclusive economic zones.

Toshiro Ino, Japan’s deputy defense minister, said the missiles “appear to have flown on erratic trajectories”.

Experts say the erratic trajectories indicate the missiles can maneuver in flight, making them more difficult to track and intercept.

The US Indo-Pacific Command said in a statement that the recent launch “highlights the destabilizing effect of the DPRK’s illicit weapons of mass destruction and ballistic missile programs,” using the official acronym for North Korea.

North Korea marked Harris’ trip to Seoul with a series of missile launches — firing short-range ballistic missiles on Sunday, Wednesday and Thursday, including just hours after the vice president left South Korea.

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

Under Seoul’s President Yoon Suk-yeol, who took office in May, the two countries have stepped up joint drills they claim are purely defensive.

Just before Harris arrived in Seoul, Washington dispatched the nuclear-powered aircraft carrier USS Ronald Reagan to South Korea for 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 modernizing its weapons and exploiting a world divided by US-China rivalry and Russia’s annexation of more Ukrainian territories,” he said.

“Pyongyang’s actions once again demonstrate the need for Washington and Seoul to step up military deterrence, tighten economic sanctions and strengthen political coordination with Tokyo,” he added.

South Korean and US officials have been warning for months that North Korean leader Kim Jong Un is preparing for another nuclear test.

On Wednesday, South Korea’s spy agency said North Korea’s next nuclear test could take place in the window between China’s upcoming party convention on Oct. 16 and the US midterm elections on Nov. 7.

North Korea, under multiple UN sanctions over its weapons programs, typically seeks to maximize the geopolitical impact of its tests through careful timing.

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