WASHINGTON (REUTERS) - North Korea freed two American citizens from prison and they were returning to the United States on Saturday after the surprise involvement of the top-ranking US intelligence official in their release.

Kenneth Bae and Matthew Todd Miller, who had been detained for months by the secretive Asian state, were being accompanied home by the director of national intelligence, James Clapper, his office said.

Their release comes less than three weeks after another American was freed by Pyongyang.

Bae, a missionary from Washington state, was arrested in North Korea in November 2012 and sentenced to 15 years hard labour for crimes against the state.

Miller, who reportedly was tried on an espionage charge, had been in custody since April this year and was serving a six-year hard labour sentence.

The United States had frequently called for the men to be freed for humanitarian reasons, especially since Bae was said to have health problems.

Bae's delighted son, Jonathan, told Reuters from Arizona that he received a call Friday night and spoke to his father.

"The brief time on the phone, he sounded good," Jonathan said. "I'm sure he will be back to his old self in no time.

"It came out of the blue. One minute he was doing farm labour and the next minute they are saying, 'You are going home.' Just like everyone else, he was surprised," he said.

The men were released just hours before President Barack Obama was to start a trip to Asia that will include talks with Chinese leaders about how Beijing can use its influence with North Korea to rein in its nuclear weapons programme, US officials have said.

"It's a wonderful day for them and their families," Obama said at the White House.

"Obviously we are very grateful for their safe return and I appreciate director Clapper doing a great job on what was obviously a challenging mission."

CLAPPER'S ROLE

Clapper's involvement in the release was rare and surprising.

An Obama administration official, who declined to be identified, said there was no connection between Clapper's visit and the issue of North Korean nuclear weapons but that he acted as a presidential envoy with a broader mandate to listen to what North Korea had to say.

Arrangements for the release had come together in the past several days and North Korea had asked for a high-ranking envoy to be involved, the official said.

Clapper went to Pyongyang but there was no indication that he met personally with North Korean leader Kim Jong Un.

A senior State Department official, who declined to be identified, said the release of Bae and Miller and Clapper's role did not constitute an opening in relations with North Korea.

The official said for that to happen, Pyongyang must fulfill its commitments on denuclearisation and human rights.

"He was not there to negotiate. And our position hasn't changed."

While it was not clear what prompted Pyongyang to release the men, North Korea has been on a diplomatic campaign to counter charges by a United Nations body that highlighted widespread human rights abuses and a move by some UN members to refer the state to an international tribunal.

The US State Department issued a statement thanking Sweden for its part in the release of Bae and Miller. Sweden serves as a diplomatic intermediary for the United States in North Korea because Washington has no diplomatic ties with Pyongyang.

INTERNATIONAL PRESSURE

Victor Cha of the Centre for Strategic and International Studies, said the release could indicate North Korea wants to press Obama on the eve of his Asian trip and that Pyongyang is feeling international heat from the UN resolution.

"This is worrying to them," Cha said.

"They have never seen anything like this before. Moreover, it is not coming from the US but from the entire international community. They are trying to blunt criticism and perhaps water down the resolution with these actions."

In late October, North Korea freed Jeffrey Fowle, 56, a street repair worker from Miamisburg, Ohio, who was arrested in May for leaving a Bible in a sailor's club in the North Korean city of Chongjin, where he was travelling as a tourist.

In September, Pyongyang allowed Bae, Miller and Fowle to be interviewed by CNN and the Associated Press. The men said they were being treated humanely and appealed to Washington to push for their release.

Miller, of Bakersfield, California, and said to be in his mid-20s, had gone to North Korea on a tourist visa, which state media said he tore up while demanding Pyongyang grant him asylum.

The Associated Press reported Miller was tried on an espionage charge and prosecutors at his trial said he had falsely claimed to have secret information about the US military stationed in South Korea.

Bae's family said on its website that Bae had been operating out of China since 2006 and had led more than a dozen tours of North Korea. They said his health problems included diabetes, an enlarged heart, deteriorating vision and back and leg pains.

Stephan Haggard, a North Korea expert at the University of California, said North Korea's policy was still zig-zagging.

"After pursuing a charm offensive aimed at restarting North-South talks and even a human rights dialogue, North Korea shut those initiatives down following the tabling of a strongly worded human rights resolution at the UN General Assembly. But the release of Miller and Bae suggests an effort to keep channels for dialogue open."