North Korea fired four cruise missiles off its east coast on Wednesday as its rivals South Korea and the United States held joint military exercises, the South Korean military said.
The military initially reported "multiple missiles" without elaborating; South Korean Defence Minister Lee Jong-sup told parliament on Thursday that there were four.
Lee added that North Korea appears to have made "substantial progress" in miniaturising nuclear warheads to fit tactical guided weapons systems.
"I don't see that they are ready yet to mount on what North Korea has recently called tactical guided weapons, but we're looking into the possibilities with the U.S.," he said.
When asked whether North Korea's nuclear weapons have come close to deployment, Lee said they have reached "substantial levels."
The missiles were fired about 10:15 a.m. (0115 GMT) from South Hamgyong province, the South's military said, just three days after the launch of a short-range ballistic missile.
Pyongyang has long bristled at exercises conducted by South Korean and U.S. forces, saying they are preparation for an invasion of the North, and it fired the missiles into the sea as the drills were underway.
South Korea and the United States say the exercises are purely defensive.
The JCS statement said that the military was on high alert and that South Korean and U.S. intelligence authorities were analysing the launches.
South Korea's Yonhap news agency said Wednesday's launches could have involved strategic cruise missiles with a potential nuclear capability, which the North tested on March 12 from a submarine.
A U.S. State Department spokesperson called on North Korea "to refrain from any further destabilising acts" and reiterated that the U.S. commitment to the defence of South Korea and Japan remained "ironclad".
The allies are set to conclude 11 days of exercises, called Freedom Shield 23, on Thursday.
"We will successfully wrap up our Freedom Shield exercise as planned under firm combined defence posture," the South Korean military said.
On Wednesday, the USS Makin, an amphibious assault ship, docked in South Korea for the allies' first large-scale amphibious landing exercise in five years, the U.S. military said.
South Korean President Yoon Suk Yeol, meanwhile, visited the military cyber command and called for proactive operations to defend against cyber threats, his office said.
North Korea has been ramping up its military tests in recent weeks, firing an intercontinental ballistic missile last week and conducting what it called a nuclear counterattack simulation against the United States and South Korea over the weekend.
It has also directed strong rhetoric against Washington and Seoul. Its state news agency quoted a foreign official as saying that pressure on Pyongyang to give up its nuclear weapons was tantamount to declaration of war.
The remark was directed at the U.S. ambassador to the United Nations, Linda Thomas-Greenfield, who on Monday called North Korea's weapons programs "unlawful" and said it should abandon them "in a complete, verifiable, and irreversible manner".
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