The US Secretary for the Army has begun an investigation in South Korea into allegations that American troops killed hundreds of civilians during the Korean War.

Louis Caldera visited No Gun Ri on Monday where American soldiers are said to have gunned down refugees in the first weeks of the war 50 years ago.

Mr Caldera, the most senior US politician to investigate the allegations, interviewed survivors who said they saw the massacre and talked to families of victims.

But Mr Caldera said the US could not investigate "every firefight, every battle" of the three-year war, despite allegations that US forces killed unarmed civilians in dozens of incidents.

Mr Caldera hinted that the investigation could take some time. A South Korean defence ministry spokesman told reporters Mr Caldera had said to him the investigation could take longer than expected.

Machine guns

Survivor Chung Ku-ho told how American soldiers huddled hundreds of refugees under a railway bridge at No Gun Ri in late July 1950 and shot them with machine guns. He said up to 400 people were killed.

During his visit to the site, Mr Caldera ran his fingers across bullet holes in the bridge and listened to an elderly witness give an account of what happened.

"I want to thank No Gun Ri survivors for being here today and for the opportunity to visit the No Gun Ri site and hear testimony to expedite the process of ascertaining the truth of the matter of what happened at No Gun Ri," Mr Caldera said.

South Korea is conducting its own separate inquiry into the allegations. The Americans hope both investigating teams will come closer during Mr Caldera's visit.

US veterans admit to killings

US troops were stationed in South Korea at the head of a United Nations coalition that defended the country after an invasion from the northern half of the divided peninsula in 1950.

Some American veterans say they were ordered in the early, messy few days of the war to attack groups of South Korean refugees because North Korean guerrillas were suspected to be hiding among them.

US veterans have appeared on television to support the accounts of survivors from No Gun Ri, the most prominent incident.

Compensation claim

Survivors and relatives of victims want compensation.

Courts in South Korea have so far rejected their claims on the grounds that the actions were filed too late.

The survivors say previous military-led governments suppressed their action because they did not want to embarrass the Americans, who remain South Korea's main ally.

Some 37,000 US troops are still stationed in South Korea to deter further attacks by the north.

Our correspondent in Seoul, Andrew Wood, says neither side wants the investigation to overshadow the commemorations of the 50th anniversary of the beginning of the Korean war in June.