North Korea fired two short-range missiles into the sea Sunday, Seoul's military said, in an apparent show of anger at an upcoming joint military drill between South Korea and the US.

The North fired two short-range ballistic missiles into the sea off the east coast at 1:20am and 1:30am (local time), the South's defence ministry told AFP.

"Their range appears to be around 500 kilometres," he said, adding Seoul's military had stepped up monitoring for additional launches.

The move - the latest in a series of similar launches in recent weeks - came a day after Pyongyang condemned an upcoming Seoul-Washington naval joint exercise.

The annual drill, to be held between July 16 to 21, involves the US aircraft carrier George Washington, which arrived in the southern port of Busan on Friday.

The North bristled at the port visit by the massive carrier, calling it a "reckless" act of provocation on Saturday.

"The US should properly understand that the more persistently it resorts to reckless nuclear blackmail and threat, the further (the North) will bolster up its cutting edge nuclear force for self-defense," said the North's top military body, the National Defence Commission.

The North has habitually slammed joint military exercises held south of the border and often responded with missile launches.

Sunday's launch was the fifth in less than two weeks.

Previous tests had preceded Chinese president Xi Jinping's state visit to Seoul, and were seen by some analysts as a show of pique at his decision to visit Seoul before Pyongyang.

In between the launches, Pyongyang has also made several peace overtures, including a proposal for both sides to halt all provocative military activity.

Seoul dismissed the offers as "nonsensical" in the light of Pyongyang's nuclear program. It also made it clear that the annual joint drills are non-negotiable.

Tokyo lodges protest

The missile launch prompted Japan to lodge a protest with Pyongyang, the Japanese government said.

"This is a clear violation of UN Security Council resolutions," Japanese chief cabinet secretary Yoshihide Suga told reporters. "The government made a protest through diplomatic channels in Beijing."

There were no damages or casualties from the firing, Mr Suga said.

North Korea's missile launch follows a pact between the two countries, in which Tokyo lifted some of its sanctions in return for Pyongyang's decision to resume a probe into the fate of Japanese citizens kidnapped by North Korean agents in the 1970s and 80s.

AFP/Reuters