Kerry said the US was delighted that Fowle was back home, and hoped North Korea would recognise that it could build goodwill and "open up a diplomatic track" between Washington and Pyongyang by releasing the other two.

He was speaking after a joint meeting of the top diplomats and defence officials of close US ally, South Korea.

Kerry's announcement came as least one North Korean legal expert suggested Washington formally apologise to Pyongyang. The suggestion by Sok Chol-won, a professor of international law, may appear ludicrous to outsiders in democracies, but it highlights how autocratic North Korea assumes that a government is responsible for its citizens' actions.

"In order to return the prisoners to their country, the United States must make an official apology and request their release," Sok, who teaches at Pyongyang's Academy of Social Sciences, said.

There are other examples of North Korea expecting foreign governments to control their society.

Earlier this year, it threatened retaliation if Washington didn't ban an upcoming Hollywood movie featuring Seth Rogen that portrays Kim Jong-un as the villain.

It also regularly insists that Seoul keep its media from reporting negatively about the North Korean leadership and block activists from floating anti-North Korea propaganda in balloons across the border.

North Korea closely regulates its academics, media and intellectuals, so Sok's comments can also be seen as a reflection of how the leadership wants to resolve the cases of Matthew Miller, who is serving a six-year jail term on charges of espionage, and Kenneth Bae, a Korean-American missionary who was sentenced to 15 years in jail for alleged anti-government activities.

Sok's advice also fits into North Korea's version of diplomacy and propaganda that aims to get a big power like the US - seen as an imperialist bully - to bow to a proud nation and say sorry for its perceived faults. "It's not a matter of individuals. It's between countries," said Ri Kyong-chol, another law professor at the academy.

"Between the US and our country there is no political channel ... If there were diplomatic relations between our two countries this kind of problem wouldn't happen."

At a time when it faces growing outside criticism over its alleged human rights abuses, North Korea would see a US apology as showing the outside world that it was justified in arresting the Americans, said Chang Yong-seok, a senior researcher at Seoul National University's Institute for Peace and Unification Studies.

Chang said North Korea would also use the apology to bolster what it says is its struggle against US hostility. Fowle was arrested for leaving a Bible in a nightclub in the city of Chongjin, where he was visiting with a foreign tour group.

North Korean state media said that Fowle was released after Kim granted him a special pardon following "repeated requests" from President Barack Obama.

Both Miller and Bae have said they believe their only chance of release is the intervention of a high-ranking government official or a senior US statesman.

In the past, former presidents Bill Clinton and Jimmy Carter have travelled to Pyongyang to bring detainees back home.