WASHINGTON (WASHINGTON POST) - Defence Secretary James Mattis said on Friday (July 27) that the Pentagon could deploy US military teams to North Korea for the first time in years to search for the remains of US 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 US troops killed in the war.

President Donald 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 US 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 US 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 US and North Korean officials.

The US military has not searched for remains in North Korea since 2005, in part because of threats North Korean officials made against US troops, US officials have said. No remains of US troops have been recovered since 2007, when a team led by former New Mexico governor Bill Richardson received the bodies of six deceased US 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 Defence POW/MIA Accounting Agency, where a laboratory 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 US officials are still determining what is possible.

Trump tweeted late on Thursday that the remains "will soon be leaving North Korea and heading to the United States," though at that point they already were in South Korea and will remain there 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."