Repatriation of American remains from the 1950-53 conflict was part of the agreement signed between US President Donald Trump and North Korean leader Kim Jong Un at their landmark summit in Singapore last month.

The US military transport plane took off from the Osan Air Base in South Korea at 5:55 am local time (2055 GMT Thursday), Yonhap said, citing a Seoul government source.

The aircraft's destination was the Kalma airport in North Korea's eastern city of Wonsan, the agency added.

"It is believed to have landed there an hour later," the official was quoted as saying. "It will return (to South Korea) today."

US Secretary of State Mike Pompeo said last week that the repatriations will begin soon, but did not confirm media reports about the first transfer of some 50 sets of remains.

The South Korean official cited by Yonhap said it was unclear how many sets of remains will be returned on Thursday.

US defence officials are expected to examine the remains in South Korea before sending them on for forensic identification in Hawaii, the agency added.

More than 35,000 Americans were killed on the Korean Peninsula during the war, out of which around 7,700 are still considered missing, including 5,300 in North Korea alone, according to the Pentagon.

Between 1990 and 2005, 229 sets of remains from the North were repatriated, but those operations were suspended when ties deteriorated over Pyongyang's nuclear weapons programme.