South Korean troops have fired warning shots at soldiers from the North amid rising tension over North Korean plans to test a nuclear weapon.
Early reports suggest that about 40 shots were fired when soldiers crossed into the demilitarised zone.
Meanwhile, South Korea has welcomed a statement by the United Nations Security Council urging North Korea to abandon plans to test a nuclear weapon.
Seoul joined in calling on the North to return to six-party talks.
Some observers have warned that a nuclear test could come as early as this weekend.
After intruding some 30 metres (yards), the Northern troops returned to their side of the military demarcation line, it added.
One Southern military source, speaking anonymously to The Associated Press, said it was unclear whether the intrusion was "intentional or whether it was to catch fish".
In its statement, the Security Council warned of unspecified action against North Korea if it went ahead with a test explosion.
KOREAN NUCLEAR CRISIS
Sept 2005:At first hailed as a breakthrough, North Korea agrees to give up nuclear activities
Next day, N Korea says it will not scrap its activities unless it gets a civilian nuclear reactor
US imposes financial sanctions on N Korea businesses
July 2006: N Korea test-fires seven missiles
UN Security Council votes to impose sanctions over the tests
Oct 2006: N Korea threatens nuclear test
A nuclear test would "jeopardise peace, stability and security in the region and beyond" and, in the event of a test, the Security Council would "act consistent with its responsibility under the Charter of the United Nations".
South Korea said it supported and welcomed the "UN Security Council's clear and strict position" against the threatened test.
"North Korea must scrap its plan for a nuclear test and return to the six-party talks immediately and without any pre-conditions," its foreign ministry said.
The BBC's Charles Scanlon reports from Seoul that a nuclear test could raise regional tension and even spark an arms race, as well as affecting the South's policy of reconciliation.
South Korea is sending its top delegate to the six-party negotiations, Chun Yung-Woo, to Beijing on Monday for talks with his Chinese counterpart Wu Dawei.
North Korea has been boycotting the six-party talks - which involve the two Koreas, the USA, China, Russia and Japan - in protest at US sanctions against it.
Japanese Vice-Foreign Minister Shotaro Yachi told Japanese TV after talks with US officials in Washington that it was possible a test could come this weekend.
"Based on developments so far, it would be best to view a test as possible this weekend," he told Japan's Asahi TV channel.