Beijing said on Thursday it had offered "humanitarian" assistance in the recent release of a US soldier from captivity in North Korea.

Travis King, 23, had been held in North Korea since July, when he crossed over the Demilitarised Zone from South Korea.

After nearly three months, he passed through the North Korean border to China with the help of Swedish diplomats, where he was handed over to the US ambassador and a senior military officer on Wednesday. He then flew out to a US military base.

And, in a rare example of US-China cooperation, Beijing confirmed on Thursday it had offered help.

"At the request of the North Korean and US sides, China has provided necessary assistance from a humanitarian perspective," foreign ministry spokeswoman Mao Ning said, when asked about the case.

US National Security Advisor Jake Sullivan on Wednesday thanked China "for its assistance in facilitating the transit of Private King".

But China did not mediate with North Korea for King's release and the United States made no concessions "full stop" to Pyongyang, a senior US administration official said. (AFP)