North Korea has not yet followed through on its promise to return the remains of American soldiers who died in the Korean War, as U.S. officials say there’s been no further discussion since the regime turned over an initial 55 boxes of remains more than six months ago.

That transfer was hailed as a major step forward, stemming from the first summit between President Trump and Kim Jong Un last summer. With a second summit looming next week in Vietnam, however, the chief scientist tasked with identifying the remains notes the 55 boxes represent only a fraction of the total remains thought to be on North Korean soil.

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“We would be delighted if we could get more remains turned over to us. But there's been no discussion ... since we received the 55 boxes,” Dr. John Byrd said in an interview with Fox News.

While Byrd is eager to get more remains from North Korea, he understands it will take some time for the details to be worked out. He estimates that remains of more than 5,000 of the roughly 7,500 Americans missing in action are still in North Korea.

'There's been no discussion ... since we received the 55 boxes.' — Dr. John Byrd, chief scientist identifying Americans' remains from North Korea

According to the text of the agreement with North Korea coming out of the first summit, "The United States and [North Korea] commit to recovering POW/MIA remains, including the immediate repatriation of those already identified."

North Korea also pledged to work “toward” complete denuclearization, something critics say the communist regime has not adequately honored either. Byrd said Pyongyang has not turned over any more remains since the dignified transfer took place in Hawaii last August.

“Soon we will know their names, and we will tell their stories of courage," Vice President Pence said during that ceremony. Pence’s father Edward, a U.S. Army second lieutenant, fought in the Korean War and returned with a Bronze Star.

The administration remains outwardly optimistic about the continuing talks with North Korea. Last year’s summit alone represented a historic breakthrough in long-frozen relations between the U.S. and the North Korean regime.

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“We’ve seen progress since the beginning of this administration. When the president first took over, we were at great odds with North Korea. We’re finally making progress. We’re not seeing missiles being tested and flying over other countries. The remains are coming home; the hostages were released,” White House Press Secretary Sarah Sanders said Friday on “Fox & Friends.”

Byrd said that “hopefully sometime in the not-too-distant future, we'll be able to complete those negotiations and proceed towards field operations [in North Korea].” He said they don’t have “certainty” on that point now.

Meanwhile, the task of identifying the remains received in the first transfer is a painstaking one.

Byrd described it as the “hardest single project” he has faced. Only three American soldiers have actually been identified to date, though the scientist said more Americans will be identified in the days ahead.

“We're in the final stages of making several more identifications sometime over the next month,” Byrd said. “Right now, we're looking at four.”

Byrd said based on DNA results, it is clear up to 80 percent of the remains turned over are Americans. Ultimately, dozens more are expected to be identified. He estimated the total number will likely be between 50 and 100.

Those identified so far by Dr. Byrd’s Defense POW/MIA Accounting Agency are: Army Master Sgt. Charles H. McDaniel, 32, of Vernon, Ind.; Army Pfc. William H. Jones, 19, of Whitakers, N.C.; and Army Sgt. Frank Suliman of New Jersey.

“On behalf of the families with loved ones missing from the Korean War, DPAA is grateful for the 55 boxes of remains that North Korea turned over," said Charles Prichard, Director of Public Affairs for the Defense POW/MIA Accounting Agency. "We appreciate the willingness of the North Koreans to stay in communication with us regarding efforts to locate and recover the estimated 5,300 sets of U.S. remains believed to be located in North Korea.”

Byrd described some of the methods used to identify the remains, including DNA testing and looking at “isotope signatures in the bones” to determine if the remains are American soldiers or those of South Korean allies serving alongside them. Scientists look for signs of staples of the American diet like corn and sugar and processed foods.

“We also look at oxygen isotopes and that comes from your drinking water, and we can relate the oxygen isotopes to places in the world that have those same characteristic patterns,” he added.

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As for what comes next, he said negotiations are ongoing: “It's going to be a complicated process. We just have to be patient with it.”