North Korea has fired at least one ballistic missile, which flew about 800km before hitting the sea off its east coast, South Korea's military says, as the isolated state stepped up its defiance of tough new UN and US sanctions.

South Korea's Yonhap news agency said the missile was likely a medium-range Rodong missile. If confirmed, it would mark North Korea's first test of a medium-range missile, capable of reaching Japan, since 2014.

The launch comes amid heightened tension on the Korean Peninsula after the North rejected UN Security Council sanctions imposed earlier in the month in response to a nuclear test conducted in January and the US issued fresh sanctions this week.

The missile was launched from north of the capital, Pyongyang, flying across the peninsula and into the sea off the east coast, the South's Office of the Joint Chiefs of Staff said.


It appeared the North may have fired a second missile soon after from the same region, with a projectile disappearing from radar at an altitude of about 17km, it added.

South Korea did not confirm the type of the missiles. But 800km was likely beyond the range of most short-range missiles in the North's arsenal.

The North's Rodong missile has an estimated maximum range of 1300km, according to the South's Defence Ministry.

The US State Department said it was closely monitoring the situation and urged North Korea to focus on taking concrete steps towards fulfilling its international commitments and obligations.

Japan quickly condemned the launch, lodging a protest with North Korea through its embassy in Beijing, Prime Minister Shinzo Abe told Parliament.

Last week, the North fired two short-range missiles into the sea off its east coast and its leader, Kim Jong Un, ordered more nuclear weapons tests and missile tests to improve attack capability.

New US sanctions on Pyongyang aim to expand its blockade by blacklisting individuals and entities that deal with the North's economy.

Analyst Lee Choon Geun at South Korea's state-funded Science and Technology Policy Institute said the North can probably place nuclear warheads on its shorter-range Scuds and medium-range Rodong missiles, which would put South Korea and Japan under its striking range. Other analysts question that.

The North began to develop ballistic missiles in the 1970s by reverse-engineering Soviet-made Scuds it acquired from Egypt. After several failures it put its first satellite into space aboard a long-range rocket launched in December 2012. Its second successful satellite launch occurred last month.

Experts say a militarised version of the rocket the North used to put its second satellite into orbit last month would potentially have the range to reach the US mainland.