North Korea fired a ballistic missile into the sea off its east coast early on Sunday, South Korea's military said, the first time the isolated state has tested such a device since U.S. President Donald Trump took office.

The missile was launched from an area named Panghyon in North Korea's western region just before 8 a.m. (2300 GMT Saturday) and flew about 500 kilometres (300 miles), the South's Office of the Joint Chiefs of Staff said in statements.

"Our assessment is that it is part of a show of force in response to the new U.S. administration's hardline position against the North," the office said.

The South's military said Seoul and Washington were analysing the details of the launch. Yonhap News Agency said the South Korean military is assessing the launch to confirm whether it was a Musudan intermediate-range ballistic missile, which has a designed flight range of 3,000 kilometres (1,800 miles).

The U.S. military also said it had detected a missile test launch by the North and was assessing it, according to a U.S. defence official in Washington.

The North tried to launch a Musudan eight times last year but most attempts failed. One launch that sent a missile 400 km (250 miles), more than half the distance to Japan, was considered a success by officials and experts in the South and the United States.

Sunday's launch comes a day after Trump held a summit meeting with Japanese Prime Minister Shinzo Abe and said he agreed to work to ensure strong defence against North Korea's threat.

South Korea's presidential Blue House said a National Security Council meeting was called and chaired by President Park Geun-hye's top national security advisor.

North Korean leader Kim Jong Un said in his New Year speech that the country was close to test-launching an intercontinental ballistic missile (ICBM) and state media have said such a launch could come at any time.

The missile launched on Sunday did not appear to be an ICBM, Yonhap cited a South Korean military source as saying.

Kim's comments prompted a vow of an "overwhelming" response from U.S. Defence Secretary James Mattis when he travelled to South Korea earlier this month.

North Korea conducted two nuclear tests and a number of missile-related tests at an unprecedented rate since early last year and was seen by experts and officials to be making progress in its weapons capabilities.