When North Korea handed over 55 boxes of bones that it said are remains of American war dead, it provided a single military dog tag but no other information that could help U.S. forensics experts determine their individual identities, a U.S. defense official said Tuesday.

The official, who discussed previously undisclosed aspects of the remains issue on condition of anonymity, said it probably will take months if not years to fully determine individual identities from the remains, which have not yet been confirmed by U.S. specialists to be those of American servicemen.

The official did not know details about the single dog tag, including the name on it, or whether it was even that of an American military member. During the Korean War, combat troops of 16 other United Nations member countries fought alongside U.S. service members on behalf of South Korea. Some of them, including Australia, Belgium, France and the Philippines, have yet to recover some of their war dead from North Korea.

The 55 boxes were handed over at Wonsan, North Korea last Friday and flown aboard a U.S. military transport plane to Osan air base in South Korea, where U.S. officials catalogued the contents. After a repatriation ceremony at Osan, the remains will be flown to Hawaii where they will begin undergoing in-depth forensic analysis, in some cases using mitochondrial DNA profiles, at a Defense Department laboratory to attempt to establish individual identifications.

Defense Secretary Jim Mattis said last week that the return of the 55 boxes was a positive step but not a guarantee that the bones are American.

"We don't know who's in those boxes," he said. He noted that some could turn out to be those of missing from other nations that fought in the Korean War. "They could go to Australia," he said. "They have missing, France has missing, Americans have. There's a whole lot of us. So, this is an international effort to bring closure for those families."

North Korea provided the 55 boxes in a delayed fulfillment of a commitment its leader, Kim Jong Un, made to President Donald Trump at their Singapore summit on June 12. Although the point of the summit was for Trump to press Kim on giving up his nuclear weapons, their joint statement after the meeting included a single line on an agreement to recover "POW/MIA remains, including the immediate repatriation of those already identified."

North Korea had told U.S. officials more than once in recent years that it had about 200 sets of U.S. war remains, although none was "already identified." It remains unclear whether the boxes provided on July 27 include all of the bones North Korea has accumulated over the years. In the past, the North has provided bones that in some cases were not human or that were additional bones of U.S. servicemen already identified from previously recovered remains.

The Pentagon estimates that of the approximately 7,700 U.S. MIAs from the Korean War, about 5,300 are unaccounted for on North Korean soil. Many were buried in shallow graves near where they fell on the battlefield; some others died in North Korean or Chinese-run prisoner of war camps.

Efforts to recover remains in North Korea have been fraught with political and other obstacles since the war ended on July 27, 1953. Between 1990 and 1994, North Korea unilaterally handed over 208 caskets to the U.S., which turned out to contain remains of far more than 208 individuals, although forensics specialists thus far have established 181 identities. In addition, a series of U.S.-North Korean recovery efforts, termed "joint field activities," between 1996 and 2005 yielded 229 caskets of remains, of which 153 have been identified, according to the Pentagon.