North Korea has test-fired several missiles towards the Sea of Japan.
South Korea's defence ministry said the launches appeared to be part of a routine military exercise by the secretive communist state.
Reports from Japan and South Korea said that the tests involved a number of short-range anti-ship missiles.
The US envoy to North Korea played down the significance of the tests, but added that Pyongyang should concentrate its efforts on nuclear disarmament.
Christopher Hill said: "We're not surprised. It's something they have done on several occasions."
He added: "What North Korea needs to do is focus on its future, and its future is getting out of the nuclear weapons business."
Japanese Prime Minister Shinzo Abe described the action as "extremely regrettable", and said it undermined international trust in Pyongyang.
Some commentators said the tests were a response to Friday's launch by South Korea of its first destroyer equipped with US-supplied high-tech Aegis radar.
North Korea's missile development, along with its nuclear weapons programme, has been a major source of concern in the region.
Last year saw the secretive state test-fire a series of missiles - including a long-range ballistic missile - before conducting its first test of a nuclear device, provoking a diplomatic storm.
In February this year, the North agreed to close its main nuclear reactor in exchange for international fuel aid and other incentives.
However, implementation of the deal has been stalled by a row over the release of North Korean funds that have been frozen in a Macau bank.
Last month, during a huge military parade in the capital, Pyongyang, North Korea displayed a newly developed ballistic missile capable of reaching the US territory of Guam.
The BBC's Charles Scanlon in Seoul says the firing of short-range anti-ship missiles has much less strategic significance - but could serve as a warning of potential trouble to come.