Shots have been fired across the border between North and South Korea, the defence ministry in Seoul has said.

The South returned fire after North Korean soldiers fired several shots at a South Korean guard post in the demilitarised zone, the ministry said.

There have been no reports of any casualties, officials said.

The two neighbours have frequently been involved in skirmishes along their tense frontier, but this is the first such incident this year.

The demilitarised zone between North and South Korea is the most heavily-fortified border in the world.

The two sides are still technically at war - having never signed an official ceasefire at the end of the Korean War in 1953.

But in recent months there has been an improvement in North Korea's ties with the outside world.

In July, it finally shut down its Yongbyon nuclear reactor, after months of negotiations.

The closure was part of an international disarmament deal under which North Korea is to receive energy aid and political incentives in return for ending its nuclear programme.