South Korea's defence chief called Friday for tight vigilance against North Korea, saying Pyongyang was preparing for a new infiltration exercise near the disputed Yellow Sea border.

"As the ice melts, North Korea is preparing to resume a seaborne infiltration drill," Defence Minister Kim Kwan-Jin told a parliamentary session.

He did not elaborate on the drill but said "surprise provocations using new means and methods are always possible".

The South's military is deploying light attack helicopters on five border islands to counter any hovercraft infiltration by special forces, the ministry said in a report to parliament.

Media reports last month said the North was building a new hovercraft base that would allow it to launch a quick invasion of the islands.

Kim said US and South Korean troops would maintain close watch on the North's military activities even after they complete their current major joint exercise.

The allies launched the annual Key Resolve/Foal Eagle drills on February 28, some three months after the North's deadly shelling of one of the five frontline islands.

They completed computerised war games on Thursday, but field training involving a US aircraft carrier will continue through next month.

The two exercises, denounced by North Korea as a rehearsal for invasion, involve a total of 12,300 US troops and some 200,000 South Korean service members including reservists.

Kim said the North had tried to disrupt the war games by beaming signals intended to jam military communications which use Global Positioning Systems.

The jamming, which began last Friday, apparently failed. Kim said the North is believed to have a number of sets of jamming equipment mounted on vehicles.

The North's artillery attack in November, which killed two marines and two civilians, prompted South Korea to strengthen island defences.

The South also accuses its neighbour of torpedoing a warship near the border in March 2010 with the loss of 46 lives, a charge Pyongyang denies.

earlier related report
Group suspends N. Korea leaflet launch after murder
Seoul (AFP) March 11, 2011 - South Korean police were Friday investigating the murder of the mother of an activist opposed to North Korea, an incident which prompted the suspension of a leaflet launch into the communist state.

Police said the 75-year-old woman surnamed Han was found dead with head injuries on Thursday afternoon, on the floor of a small store which she ran in northern Seoul.

Lee Byoung-Woo, chief detective at Gangbuk district police station, said they are treating the case as murder but have no suspects yet.

"We cannot confirm anything about the possibility of North Korea's involvement," Lee told AFP in answer to a question, adding that nothing was stolen from the store.

Han was the mother of Chu Sun-Hee, one of a group of activists intending to launch tens of thousands of anti-regime leaflets into the North on Saturday.

Pyongyang has threatened to open fire with artillery to disrupt such launches from Imjingak, a tourist area overlooking the closely guarded border.

Chu is a senior official of a group called the Korea Parent Federation.

"We're waiting for the results of the police investigation," federation chief Lee Hung-Woo told AFP.

"We hope police will launch a thorough investigation to see whether this is a simple homicide or whether North Korean agents or leftist groups were behind it."

Some left-leaning activists had vowed to try to block the leaflet launch, which they said would aggravate tensions with the North.

Park Sang-Hak, head of a defector group called Fighters for Free North Korea, said the launch had been suspended until after the funeral service.

"It's just a temporary suspension and we will send leaflets once the funeral has been finalised," he said.

Park's group and other activists had planned to launch 200,000 leaflets plus DVDs and USB flash drives, suspended under helium-filled balloons designed to float over border fences.

The leaflets and electronic devices were to contain news of the uprising in Libya -- a topic seen as especially sensitive in the North -- and other material critical of the regime.