By Ian Simpson

WASHINGTON (Reuters) - The U.S. National Park Service is proposing to limit the commemorative items it keeps from the Vietnam Veterans Memorial as it faces a potential space crunch from more than 400,000 tribute objects, the agency said on Tuesday.

About 3 million people a year visit the two-acre (0.8 hectare) site on Washington's National Mall. It is best known for the sunken Vietnam Veterans Memorial Wall, where the names of more than 58,000 service personnel who died from 1959 to 1975 are engraved.

Among the items left behind since the memorial was dedicated in 1982 are a custom-built Harley-Davidson motorcycle, a general's stars, eyeglasses, military ribbons and medals, money and flags.

The National Park Service said in a statement that it was seeking public comment about changing the criteria for items it would keep in its Vietnam Veterans Memorial Collection. The deadline for submitting comments is March 10.

“By refining the scope of the Vietnam Veterans Memorial Collection, we can ensure that our energy and resources will preserve items with a direct and specific relationship to veterans of the Vietnam War,” said Gay Vietzke, superintendent of the National Mall and Memorial Parks.

The proposed changes call for the National Park Service to keep only personal artifacts of personnel whose names appear on the memorial, Vietnam War military service items, and protest and advocacy materials related to the war.

Most of the items left at the site now have no direct connection to Vietnam veterans or the war, the agency said. In cases where the collection has a large sample of a particular type of item, only a representative number will be kept.

Mike Litterst, a National Park Service spokesman, said items from the memorial were kept at a site in Maryland that houses objects from about 40 parks in the Washington region.

"While it is not currently an issue, if the collection were to continue to grow at its current pace, space would eventually become a problem and this is a pro-active effort find a solution," he said in an email.

The agency said the Vietnam Veterans Memorial Fund was raising money for an educational center where 4,000 to 6,000 articles would be put on display.

The National Mall site includes the Three Servicemen Memorial and the Vietnam Women's Memorial.