The Cheonan sank after an as-yet unexplained blast on 26 March

South Korea's navy has fired warning shots at a patrol boat from the North, the most serious skirmish since a Southern ship was sunk on 26 March.

Two vessels had violated a disputed border known as the Northern Limit Line, South Korea's military said.

"Two patrol boats crossed on two separate occasions and warning shots were fired," an official said.

Tensions have been high since a South Korean warship mysteriously sank on March 26, killing 46 sailors.

A North Korean patrol boat sailed 2.8km (1.6 miles) into South-controlled waters on Saturday, said Seoul's Joint Chiefs of Staff.

It retreated after a South Korean ship broadcast a warning, reports say.

Less than an hour later, another North Korean patrol boat violated the border, but returned to its waters after another warning was broadcast.

Two shots were fired from the South Korean vessel, a Joint Chiefs of Staff official said. There were no injuries, he said.

The border skirmish took place close to where the Cheonan, a Southern warship, sank in March, apparently struck by a torpedo. That incident left 46 sailors dead.

South Korea has not officially blamed the North, but Pyongyang has come under suspicion over the suspected attack.

An international team of investigators is expected to release its findings this week.

The area was also the scene of fatal naval clashes in 1999 and 2002, and a North Korean patrol boat was set ablaze in a firefight last November.