South Korean troops attacked the North a year before the Korean war broke out, researchers have claimed in the latest disturbing revelation about the conflict which almost led to global war.

More than 250 guerrillas from the South are said to have launched an attack on North Korean villages along the east coast in June 1949. Some reached the town of Wonsan, although all but 50 were killed in two weeks. The incident has been confirmed by a South Korean army official.

It was only one of numerous small-scale conflicts instigated by the South as well as by the North, according to a new book by Professor Kim Kwi-ok of Seoul National university.

The story supports the argument of some historians that the North's invasion of the South in June 1950 must be seen in the context of a steady build-up of hostilities from both sides.

It will add to public concern in Seoul after a string of recent revelations about war atrocities committed by South Korean and US forces in the south early in the Korean war.

Last week, the South Korean president, Kim Dae-jung, urged the US to conduct a thorough inquiry into the alleged mass killing of Korean civilian refugees by US soldiers. He told the US army secretary, Louis Caldera, who was visiting Seoul, that the "truth should be clearly brought out so that South Korea-US relations should not be damaged and will instead be enhanced".

Mr Caldera's visit to South Korea follows a recent investigation by the Associated Press news agency into a number of horrifying incidents. Its reports have shattered the conventional picture that all the atrocities in the Korean war were committed by the North Koreans or their Chinese allies.

US veterans interviewed by AP said they machine-gunned hundreds of helpless civilians under a railway bridge at No Gun Ri on July 26, 1950.

A week later, according to other veterans, a US general ordered the destruction of two strategic bridges across the Naktong River killing hundreds of civilians.

"It was a tough decision," wrote Hobart Gay, the 1st Cavalry Division commander, in a now-declassified document, "because up in the air with the bridge went hundreds of refugees."

The AP investigation documents other incidents in 1950-51 when US jets repeatedly attacked groups of Koreans in civilian clothes on the suspicion that they harboured enemy infiltrators.

In one strike, US firebombs are said to have killed 300 civilians trapped in a cave. Some pilots expressed concern that they were machinegunning innocent people.

Korean newspapers have called for a full investigation of all allegations. The defence ministry in Seoul is reported to have heard of nearly 40 similar cases of alleged civilian killings by US forces.

Korean commentators say that the incidents have long been known. But while South Korea was under military dictatorship the victims and their family members had to keep silent, fearing punishment if they spoke out.

Allegations occasionally surfaced in the US but the Pentagon always denied that there were grounds for inquiry. The situation only changed when an AP reporter, Charles Hanley began to investigate the No Gun Ri story in 1997.

US veterans who had lived with their secret for nearly half a century started to come forward. One of the most convincing witnesses was Ed Daily, now living in rural Tennessee, who was a 19-year-old volunteer with the 1st Cavalry division at the start of the Korean war.

He described how his unit surrounded several hundred civilians and penned them under a railway arch. Sporadic fire then came from under the arch, and his unit was ordered to "shoot them all."

They fired for more than 30 minutes: Mr Daily says he saw women and children and old men in his sights. He claimed up to 200 people had been killed by the time it was over.

"It started to torment many of us," Mr Daily told a television interviewer last month. "By the 1980s we started contacting one another, and we were all having the same mental problems. Over the years I went hearing a baby cry, or a woman scream, [and] it triggers a flashback."

Historically, the US has opposed attempts to establish a permanent international war crimes investigation network, and remains opposed to any system which would make the US military answerable to any other jurisdiction.

But last October President Bill Clinton and the defence secretary, William Cohen, ordered a full scale investigation under the US army inspector-general, Lieutenant General Michael Ackerman and a seven-man advisory group.

Some US veterans admit the atrocities but argue that they were inevitable in the heat of war. Some claim they were partly justified because North Korean agents may have been concealed among genuine civilians. They also say that the US troops, who were in disorganised retreat at the time, were pitifully unprepared.

Others point out that many more Korean civilians were killed by US bombing later in the war, particularly during the saturation bombing of Pyongyang in 1952.

Official war histories record that 10,000 litres of napalm and 697 tons of bombs were dropped, resulting in the deaths of almost 8,000 people.

It is unclear how far the US government is willing to investigate beyond the No Gun Ri allegations. Last week Mr Caldera said that the US will not investigate every claim that there was a loss of innocent lives. "If you begin down that path you'll never end because all war by definition is extremely violent," he said.

But a US embassy spokesman said later that further investigation had not been ruled out.

North Korea, whose own atrocities during the war are well documented, last week seized on the issue to accuse the US of a cover up.

In a separate revelation, the newspaper Hankook Ilbo in Seoul has alleged that some 1,800 political prisoners held in Taejon, south of Seoul, on charges of being communist sympathisers were executed in July 1950 by South Korean military police.

It also noted that North Korean troops later massacred a large number of rightwing prisoners in revenge.

"Our government is now urged to ascertain the number of prisoners, irrespective of ideology, massacred during the war and to identify them," the Korean Times wrote in an editorial. "In addition, the government is called upon to reveal who was in a position to order the terrible massacres."