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 exercise between South Korea and the United States.

The North fired the two ballistic missiles into the East Sea (Sea of Japan) at 1:20 and 1:30 am local time, the South's defence ministry spokesman told AFP.

"Their range appeared to be around 500 kilometres (311 miles)," 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, from July 16-21, involves the US aircraft carrier George Washington, which arrived in the southern port of Busan on Friday.

The North bristled Saturday at the nuclear-powered carrier visiting the port, calling it a "reckless" act of provocation.

"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-defence," said the North's top military body, the National Defence Commission.

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

UN resolutions bar it from conducting any ballistic missile tests. Sunday's launch -- the fifth in just over two weeks -- took place in a sensitive area near the heavily-fortified border with the South, the defence ministry spokesman said without elaborating.

Yonhap news agency said the missiles were launched only about 20 kilometres north of the Demilitarised Zone that has divided the peninsula since the 1950-53 Korean War ended in an armistice.

The North appears to have moved them from a military base about 50-60 kilometres away by using mobile launchers, Yonhap said, citing an unnamed Seoul army official.

- Kim wants to look 'bold' -

The launch area may fall within the range of South Korean artillery, said Kim Jung-Bong, a political science professor at Hanzhong University, adding the move was aimed at portraying the North's leader Kim Jong-Un as a "bold leader with guts".

"The North appears to be stepping up its threats by showing that it can fire missiles at any time and any place it wants," said Kim.

The North has often fired short-range missiles or rockets into the sea to express anger at perceived provocations.

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 make Seoul rather than Pyongyang his first stop on the peninsula.

Japan protested to North Korea over Sunday's launch via its embassy in Beijing, Japan's Kyodo News and Jiji Press said.

But Prime Minister Shinzo Abe told reporters the launch would not affect ongoing talks to try to solve the issue of Japanese abductees in the North, according to Jiji.

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

The South dismissed the offer as "nonsensical" in the light of the North's nuclear weapons programme and reiterated that the annual joint military drills with the US are non-negotiable.

But it accepted another offer by Pyongyang to send a delegation of cheerleaders to support North Korean athletes during the September 19-October 4 Asian Games at Incheon in the South.