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North Korea fired two ballistic missiles early on Saturday, South Korea's military said, Pyongyang's fourth launch this week.

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

US Vice President Kamala Harris was in Seoul on Thursday and toured the heavily fortified Demilitarized Zone that divides the peninsula.

South Korea's military said it had "detected two short-range missiles between 0645 and 0703 fired from the Sunan area in Pyongyang into the East Sea," 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 speed of Mach 6," Seoul's Joint Chiefs of Staff said in a statement, calling the launches "a serious provocation."

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

Toshiro Ino, Japan's Vice Defense Minister, said the missiles "appear to have flown in irregular trajectories." Experts say the irregular trajectories indicate the missiles are capable of maneuvering in flight, making them harder to track and intercept.

The US Indo-Pacific Command said in a statement that the latest launch "highlights the destabilizing impact of the DPRK's unlawful WMD and ballistic missile programs."

28,500 US troops stationed in South Korea

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

The US has about 28,500 troops stationed in South Korea. Under South Korea's President Yoon Suk-yeol, who took office in May, the two sides have boosted joint exercises, which they claim are purely defensive.

Just before Harris arrived 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. 

North Korea's next nuclear test

South Korean and US officials have 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 October 16 and the US midterm elections on November 7.

North Korea has tested nuclear weapons six times since 2006, most recently in 2017.

Yoon on Saturday warned of dire consequences if Pyongyang uses nuclear weapons against Seoul.

"If North Korea attempts to use nuclear weapons it will face a resolute and overwhelming response from our military and Korea-US alliance," he said in a speech marking Armed Forces Day.