North Korea fired two short-range missiles into the sea today, South Korea's military said, the first weapons launches in more than two months.
It was an apparent pressuring tactic aimed at Washington as North Korean and US officials struggle to restart nuclear negotiations.
The South's Joint Chiefs of Staff said the missiles that were fired from near the eastern coastal town of Wonsan flew about 430 kilometres before landing in the waters off the country's east coast.
A South Korean defence official, requesting anonymity because of department rules, said that an initial South Korean analysis showed both missiles were fired from mobile launchers and flew at a maximum altitude of 50 kilometres.
The North is unhappy with planned US South Korean military drills that it says are an invasion preparation, and the missile tests may be aimed at sending a message to Washington about what would happen if diplomacy fails.
The timing was also interesting, coming not long after many in the United States were focused on testimony before Congress by Robert Mueller, the former special counsel, about his two-year probe into Russian election interference.
And a day earlier, US national security adviser John Bolton left Seoul after agreeing with South Korean officials to boost cooperation to achieve North Korea's denuclearization.
But the missiles' relatively short flight distance also suggests the launches were not a major provocation, such as a test of a long-range missile capable of reaching the US mainland, and that North Korea doesn't appear to be pulling away from US-led diplomacy aimed at curbing its nuclear program.
"North Korea appears to be thinking its diplomacy with the US isn't proceeding in a way that they want. So they've fired missiles to get the table to turn in their favour," said analyst Kim Dae-young at the Korea Research Institute for National Strategy.
In recent days, North Korea has been pressuring the US and South Korea to scrap their summertime military drills.
Last week, the North said it may lift its 20-month suspension of nuclear and long-range missile tests in response to the drills.
Trump has considered the weapons moratorium a major achievement in his North Korea policy.
Some experts say North Korea's recent actions are an attempt to get an upper hand ahead of the possible resumption of talks.
North Korea wants widespread sanctions relief so it can revive its dilapidated economy, but US officials are pushing the country to take significant disarmament steps before they give up the leverage provided by the sanctions.
A senior US official said the Trump administration was aware of the reports of a short-range projectile launched from North Korea.
The official, who spoke on condition of anonymity to provide a response, said the administration had no further comment at this time.
South Korean Defence Ministry spokeswoman Choi Hyunsoo urged Pyongyang to stop acts that are "not helpful to efforts to ease military tensions on the Korean Peninsula."
If North Korea fired ballistic missiles, it could have ramifications because UN Security Council resolutions ban the North from engaging in any launch using ballistic technology.
Still, the UN Security Council has typically imposed fresh sanctions on North Korea only when it conducted long-range ballistic missile tests.
"If they were ballistic missiles, they violate the UN resolutions, and I find it extremely regrettable," Japan's Defence Minister Takeshi Iwaya told reporters in Tokyo.
It was the first such launch since Seoul said North Korea fired three short-range missiles off its east coast in early May.
Many experts said at the time that those missiles bore a strong resemblance to the Russian-designed Iskander, a short-range, nuclear-capable ballistic missile that has been in the Russian arsenal for more than a decade.
Analyst Kim Dong-yub at Seoul's Institute for Far Eastern Studies said the latest North Korean missiles could be Scud-C ballistic missiles or KN-23 surface-to-surface missiles, a North Korean version of the Iskander.
South Korea's military said it and the US military were analysing details of today's launches.
South Korea said it was monitoring possible additional launches by North Korea.
During a third summit at the Korean border late last month, Trump and North Korean leader Kim Jong Un agreed to resume nuclear negotiations, which had been deadlocked since their second summit in Vietnam in February ended without an agreement because of disputes over US-led sanctions.
Both the launches in May and today won't end that weapons test moratorium, which applies to firing intercontinental ballistic missiles.
On Tuesday, North Korean state media said Kim inspected a newly built submarine and ordered officials to further bolster the country's military capabilities.
The Korean Central News Agency said the submarine's operational deployment "is near at hand."
After analysing North Korea-dispatched photos of the submarine, experts said it likely has three or more launch tubes for missiles.
South Korean government documents say North Korea has about 70 submarines.
But most of them have only torpedo not missile launch tubes, except for a test platform with a single launch tube the North has used when it fired ballistic missiles, according to Kim Dae-young, the analyst.
The construction of such a new submarine suggests North Korea has been increasing its military capability despite nuclear diplomacy that it began with the United States early last year.
The latest launches came amid a recent flare-up of tensions on the Korean Peninsula after South Korean fighter jets on Tuesday fired hundreds of warning shots to drive away a Russian reconnaissance plane that Seoul says violated its airspace.
Before that alleged intrusion, Seoul said Russian and Chinese warplanes including the reconnaissance aircraft made an extremely unusual joint entrance into South Korea's air defence identification zone, prompting South Korean military jets to scramble.