SEOUL (AFP) - North Korea fired four short-range projectiles towards the sea on Wednesday in the latest of a series of missile, rocket and artillery tests, the South's defence ministry said.

The latest launches began at 7.30am at a site near Mount Myohyang, north-east of Pyongyang, with two fired in the morning and another two in the afternoon, all towards the Sea of Japan (East Sea), the ministry said.

"The type of the projectiles is not known," a ministry spokesman told AFP. "The test of two projectiles launched in the morning appeared to be unsuccessful because they just flew for several seconds, while two other projectiles fired in the afternoon travelled for up to 210km," he said.

The last one fired at 6pm may have fallen in an inland area after flying about 130km, the spokesman said. The ministry declined to confirm a Yonhap news agency report that the North might have tested new 300mm multiple rocket launchers in an effort to increase their range. It was the first time such a projectile has travelled more than 200km, Yonhap said.

Large-calibre rockets pose a potential threat to South Korea as they could strike key facilities far south of Seoul.

North Korea, which is known to have about 5,000 launchers, has been trying to equip them with strong guidance systems and boost their accuracy, Yonhap said.

Wednesday's exercise came as cross-border military tensions run high following a series of missile, rocket and artillery launches in recent weeks.

North Korean leader Kim Jong Un guided a missile-firing drill last Saturday after the United Nations Security Council condemned Pyongyang for its recent ballistic missile tests in violation of UN resolutions.

The North often fires missiles and rockets as a show of force or to express anger at perceived provocations, but the frequency of the recent tests is unusual.

Pyongyang's recent missile launches were carried out at locations increasingly close to the border with the South - a move analysts say is aimed at stepping up threats against Seoul.

UN resolutions bar North Korea from conducting any launches using ballistic missile technology.

But the North has defended the missile launches as a legitimate exercise in self-defence and a response to US war manoeuvres.

North Korea is ready to "destroy the enemies without mercy so that not a single man can survive to sign a document of surrender when the US imperialists ignite another Korean war", its official Korean Central News Agency said in a commentary on Wednesday.

Pyongyang has been playing hawk and dove in recent weeks, mixing its tests with peace gestures that have been largely dismissed by Seoul.

The two Koreas are currently trying to sort out logistics for the North's participation in the Asian Games, which begin in September in the South Korean city of Incheon.

Talks have broken down with the North accusing the South of duplicity and arrogance, and threatening to boycott the event.