The South's joint chiefs of staff said its troops returned fire after the North shot at an observation post in the Demilitarized Zone, the divided peninsula's fortified frontier.

The firing -- the last such incident was in November 2001 -- occurred as the United States and China sought to coax North Korea back into talks.

Beijing said a Chinese envoy just back from Pyongyang would visit Washington. A US official said earlier the North may be ready to talk about its atomic weapons plans.

The shooting could be seen as a precursor to heightened tensions on the world's last Cold War border, days from the 50th anniversary of the July 27 armistice that ended the Korean War.

But the North has in the past raised tensions to attract attention or before climbing down to a compromise or concession.

"Everything that they have done on the DMZ over the course of the past few years has been done with a particular purpose," said Korea expert Scott Snyder of the Asia Foundation in Seoul.

"The North Koreans have continued to look for ways to remind the United States that it is out there and that they can do damage as a way of trying to draw attention."

In Washington, a State Department official said North Korea appeared ready to resume three-way talks with China and the United States. After an inconclusive round in April, the North reverted to insisting on direct US talks.

In Tokyo, top government spokesman Yasuo Fukuda told reporters: "There may be a chance that things could come together in the direction of being able to have them. But I can't say anything about what form they might take."

Colonel Lee Hong-ki, a spokesman for the South Korean military chiefs, told reporters it was not yet clear what type of machinegun had been used in the incident. The joint chiefs and the US-led United Nations Command were investigating.

Lee said North Korea fired four shots in a single burst at a South Korean post. The South answered with a warning and returned fire with 17 shots, a minute after the North.

No one was wounded in the South. There was no immediate comment from the North.

The navies of the two Koreas fought along their disputed maritime border in June 2002. Six South Korean sailors and an estimated 13 Northern seamen died.