A U.N. honor-guard member carries remains during a ceremony at Osan Air Base in South Korea on Friday, as the United States received the remains of an estimated 55 service members killed during the Korean War. (Senior Airman Kelsey Tucker/Defense Department/AFP)

Defense Secretary Jim Mattis said Friday that the Pentagon could deploy U.S. military teams to North Korea for the first time in years to search for the remains of U.S. troops who were killed during the Korean War, a step that would mark a continued thawing of relations with Pyongyang.

Mattis raised the possibility after a C-17 plane flying out of North Korea landed at Osan Air Base in South Korea with what are thought to be the remains of 55 U.S. service members killed in the war. President Trump and North Korean leader Kim Jong Un agreed on the repatriation in their June 12 summit in Singapore, but negotiations had been bogged down for weeks.

Mattis, speaking at the Pentagon, called North Korea’s decision to turn over the remains a “humanitarian act” that “sets a positive environment and a positive tone for other things” as the United States seeks to get Kim’s government to get rid of its nuclear weapons.

“We have families that when they got the telegram have never had closure,” Mattis said, referring to how the U.S. government notified relatives that their loved ones were killed or missing in North Korea. “They’ve never gone out and had the body returned. What we’re seeing here is an opportunity to get those families closure and make certain that we keep looking for those remaining.”

The Pentagon estimates that the remains of about 7,700 U.S. troops are unaccounted for from the Korean War, including about 5,300 believed to be in North Korea. Repatriation efforts have occurred in fits and starts for decades in North Korea, with a mixed record of cooperation from Pyongyang and distrust between U.S. and North Korean officials.

The U.S. military has not searched for remains in North Korea since 2005, in part because of threats North Korean officials made against U.S. troops, U.S. officials have said. No remains of U.S. troops have been recovered since 2007, when a team led by former New Mexico governor Bill Richardson (D) received the bodies of six deceased U.S. troops during a private diplomatic effort.

Mattis said the remains would be reviewed in South Korea “for any anomalies” and then transported to the Pentagon’s Defense POW/MIA Accounting Agency, where a lab will conduct forensics testing. In the past, the North Koreans have been accused of deliberately mixing in animal bones in remains handed over.

“We have no indications that anything is amiss, but you don’t know and can’t confirm one way or the other,” he said. “That’s why we go through all the forensics.”

The Pentagon would “obviously like to continue” repatriation efforts, Mattis said, but U.S. officials are still determining what is possible.

Trump tweeted late Thursday that the remains “will soon be leaving North Korea and heading to the United States,” though at that point they were already in South Korea, where they will remain for the initial review.

“After so many years, this will be a great moment for so many families,” Trump tweeted. “Thank you to Kim Jong Un.”