North Korea fired at least two projectiles early on Thursday from its eastern coast, which Japanese government sources confirmed were short-range ballistic missiles, said Kyodo news.
The projectiles launched from near Wonsan flew about 430km to the east, South Korea's Joint Chiefs of Staff said. A Japanese government source told Kyodo the missiles did not reach Japan's exclusive economic zone and had no impact on Japan's national security.
The firing of ballistic missiles would cast new doubts on efforts to restart denuclearisation talks after US President Donald Trump and North Korean leader Kim Jong Un met at the demilitarised zone (DMZ) at the end of June.
The White House, Pentagon and US State Department did not immediately respond to requests for comment.
A senior US administration official said: "We are aware of reports of a short-range projectile launched from North Korea. We have no further comment."
Mr Trump's national security adviser John Bolton, who has taken a hardline towards North Korea, made no mention of the launches in a tweet on Thursday morning after a visit to South Korea, referring only to "productive meetings" with South Korean officials on regional security and building a stronger alliance.
The United States and North Korea vowed to soon hold new rounds of working-level talks, but Pyongyang has since sharply criticised upcoming joint military drills by US and South Korean troops.
North Korea's Foreign Ministry said earlier this month that Washington's pattern of "unilaterally reneging on its commitments" by holding military exercises with South Korea was leading Pyongyang to reconsider its own commitments to discontinue tests of nuclear weapons and intercontinental ballistic missiles.
"North Korea is clearly upset that the US and South Korea are conducting joint military exercises," said Harry Kazianis of Washington's Center for the National Interest.
"We should not be shocked by this move and, in fact, we should have seen it coming."
North Korea's weapons testing in May included short-range missiles as well as smaller rockets. At the time, Mr Kim oversaw the first flight of a previously untested weapon - a relatively small, fast missile that experts believe will be easier to hide, launch and manoeuvre in flight.
On Tuesday, state news agency KCNA reported that Mr Kim inspected a large, newly built submarine, accompanied by missile programme leaders. It potentially signalled continued development of a submarine-launched ballistic missile (SLBM) programme.
Denuclearisation talks between North Korea and the United States have stalled after a second summit between Mr Trump and Mr Kim in Vietnam in February broke down.
Mr Trump has repeatedly stressed his good relationship with Mr Kim and is keen for a big foreign policy win as he campaigns for re-election in 2020.
When Mr Trump and Mr Kim met last month he said they had agreed to resume working-level talks stalled since their failed summit in Hanoi in February.
On Monday, Mr Trump stressed North Korea's freeze in testing of nuclear weapons and long-range missiles, which has been in place since 2017, and positive recent exchanges.
"There was a little correspondence recently," he said. "We had very positive correspondence with North Korea. Again there's no nuclear testing, there's no missile testing, there's no nothing."
US Secretary of State Mike Pompeo last week said the State Department had "a number of conversations with the North Koreans" and he hoped the talks with North Korea would begin soon.