South Korea has requested more information from the U.S. government about a 1950 letter in which the U.S. ambassador to Seoul told the State Department that American soldiers would shoot refugees approaching their lines.

The letter was dated the day of the U.S. Army's mass killing of South Korean refugees at No Gun Ri during the 1950-53 Korean War. It is the strongest indication yet that such a policy existed for all U.S. forces in Korea, and the first evidence that the policy was known to upper ranks of the U.S. government.

The letter, declassified in 1982, is discussed in a new book by American historian Sahr Conway-Lanz, who discovered the document at the U.S. National Archives, where the Associated Press also obtained a copy.

A Foreign Ministry official, speaking on condition of anonymity in line with policy, said the South Korean government has asked Washington about the existence of the letter from Ambassador John J. Muccio to Assistant Secretary of State Dean Rusk.

"We're doing various checks about the report," the official said.

The letter detailed decisions made at a high-level meeting in South Korea on July 25, 1950, the night before the 7th U.S. Cavalry Regiment shot the refugees at No Gun Ri.

The No Gun Ri killings were documented in a Pulitzer Prize-winning story by The Associated Press in 1999, which prompted a 16-month Pentagon inquiry.

The Pentagon concluded that the No Gun Ri shootings were "an unfortunate tragedy" _ "not a deliberate killing."

It suggested panicky soldiers, acting without orders, opened fire because they feared that an approaching line of families, baggage and farm animals concealed North Korean troops.

Following reports of the Muccio letter, the No Gun Ri victims' group condemned the U.S. government for allegedly covering it up, calling for a reinvestigation by the United Nations.

In 2001, the United States offered to set up a scholarship fund and build a memorial for the victims, but the project hasn't been implemented yet due to a dispute with the victims' group.

After years of delay, Washington proposed in April to spend $4 million for the project. But the victims' group rejected the offer again, claiming Washington is trying to build a memorial for all Korean War victims killed in similar cases, not specifically for No Gun Ri victims.

The survivors claim that the U.S. government is pressuring Seoul to persuade them to accept the proposal, by saying the project's budget will expire at the end of September.

Estimates vary on the number of dead at No Gun Ri. U.S. soldiers' estimates ranged from under 100 to "hundreds." Korean survivors say about 400, mostly women and children, were killed at the village 100 miles southeast of Seoul.

Hundreds more refugees were killed in later, similar episodes, survivors say.