North Korea has fired two short-range ballistic missiles into the sea, South Korea's military has said.
It said the missiles, launched off the east coast, flew some 500km (310 miles) and fell into the water.
On Wednesday, North Korean leader Kim Jong-un claimed scientists had developed nuclear warheads small enough to fit on ballistic missiles.
However, South Korea's defence ministry said it thought the North had "not yet secured miniaturised nuclear warheads".
The claim is critical as without miniaturisation Pyongyang cannot put its nuclear weapons on missiles - an ability many analysts think may still be several years away for North Korea.
In response to the miniaturisation claims, US State Department spokesman John Kirby said of Kim Jong-un that "the young man needs to pay more attention to the North Korean people and taking care of them, than in pursuing these sorts of reckless capabilities".
The North has stepped up its bellicose rhetoric in response to the UN imposing some of its toughest sanctions.
The move by the UN Security Council came after the North conducted its fourth nuclear test in January and last month launched a satellite. Both actions contravened existing sanctions.
'Indiscriminate' strikeThe two missiles were fired from Hwanghae Province on Thursday, the South Korean military said. It added that the missiles later fell into the sea off North Korea's east coast, without providing further details.
Japan has already lodged a protest to North Korea via its embassy in Beijing, over the latest missile launch, reports Kyodo news agency.
Pyongyang has so far made no public comment on the issue.
Last week, North Korea fired six rockets into the sea, using a multiple launch system.
Tensions have been especially high this week as US and South Korean forces hold their largest ever annual joint military exercises known as Key Resolve and Foal Eagle.
The drills routinely generate tension, and Pyongyang has recently threatened to launch an "indiscriminate" nuclear strike on the US and South Korea.
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