Some 7,699 American service members are listed as unaccounted-for from the conflict. The Defence POW/MIA Accounting Agency identifies remains from battlefields around the world.
Three of the five soldiers were reported missing in 1951, 52 and 53, a fourth is known to have died in 1951 and the fifth was taken prisoner and died there, website Nu.nl said on Wednesday.
No additional information was given to help US forensics experts determine who is in those boxes, according to the official.
Byrd told reporters that "there was a single dog tag (US soldier's identity tag) provided with the remains".
The two discussed various bilateral issues, officials said, but neither Beijing, which supported North Korea in the war, nor Seoul confirmed whether they discussed a declaration ending the war. In addition, a series of US-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.
The returned material also included military hardware and uniforms, including helmets, water bottles and boots, he said.
The Pentagon estimates that almost 7,700 USA troops are unaccounted for from the war; among them are 5,300 believed to have been killed north of the 38th parallel, which largely follows the boundary between North and South Korea.
But even if he does, Sung-Yoon Lee, a Korea expert at the Fletcher School of Law and Diplomacy at Tufts University, was sceptical that there would be any fresh signs of progress when it came to denuclearisation.
The 55 boxes, draped in the blue and white flag of the United Nations, are each small enough to be carried in one person's arms.More news: Disney is officially buying Fox studios for $71 billion
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He added that the remains could be of Australians, French or any of the others who fought alongside American troops in the Korean War.
Decades after the end of the Korean War in 1953, the remains of dozens of presumed US war dead were on their way Wednesday to Hawaii for analysis and identification.
USA officials also are eager to resume searches in North Korea for more remains, an effort that has been stalled for more than a decade due to rising nuclear tensions.
A repatriation ceremony for the remains is set for Wednesday.
Image copyright Getty Images Image caption There are thought to be around 5,300 remains of USA soldiers in North Korea Does North Korea have any others collected?
North Korea also called the talks with a delegation led by Secretary of State Mike Pompeo in July "regrettable", claiming the US has undermined the spirit of the summit.
Numerous fallen service members died in North Korea and were buried by their comrades where they fell.
The U.S. State Department said on Tuesday it expects Pyongyang to keep its commitment made at the June summit to give up its nuclear arms which it had developed for years in defiance of U.N. Security Council sanctions.