U.S. President Barack Obama delivered a personal letter to North Korean leader Kim Jong-un via his national intelligence director sent to the communist nation to win the release of two American citizens, a senior U.S. official said Sunday.

The "brief letter" stated that Director of National Intelligence James Clapper was sent in his capacity as Obama's personal envoy to obtain the release of the two Americans, the senior official told reporters aboard Air Force One before they took off for Asia, according to a White House pool report.

Clapper held talks with senior North Korean officials, but did not meet with Kim, the official said.

On Saturday, Clapper brought the two American detainees -- Kenneth Bae and Matthew Todd Miller -- home.

Bae, a Korean-American missionary, had been serving 15 years of hard labor after being detained in late 2012 for unspecified anti-state crimes. Miller was detained in April and sentenced to six years of hard labor for committing "hostile" acts.

According to the senior administration official, the North Koreans had requested that a senior American official to visit the country when they called the United States several weeks ago to float the possibility of releasing the two.

The White House chose Clapper for the mission because of his background in Korean issues and because he was a national security official, not a diplomat, which kept it out of the realm of diplomacy, the official said.

Clapper went with the "sole purpose" of obtaining the release of the two, the official said.

"It was not to pursue any diplomatic opening," he said. 

During talks with the North Korean officials, Clapper reaffirmed the U.S. position that the North should demonstrate its denuclearization commitment before nuclear negotiations reopen. The official said he did not know what issues the North Koreans raised, but that he expected they broached matters beyond the two Americans.

The U.S. informed South Korea and Japan in advance of Clapper's trip to the North, he said.

"This was a very unique opportunity to bring home two Americans," he said.

The North's decision to release the two Americans came amid international efforts to adopt a U.N. General Assembly resolution that calls for referring the totalitarian regime to the International Criminal Court (ICC) for human rights violations.

The release of the two could be part of efforts to tone down the resolution.

The North's decision could also be an olive branch aimed at restarting the stalled negotiations on its nuclear program. U.S. officials have cited the detention of American citizens as a big stumbling block to improved relations between the two countries.

Six-party denuclearization talks, which bring together the two Koreas, China, Japan, Russia and the U.S., have been stalled since late 2008. Pyongyang has called for unconditional resumption of the talks, but the U.S. has demanded the North first demonstrate its denuclearization commitment.

Obama left for China early Sunday for an annual summit of Asia-Pacific nations and a state visit to Beijing for summit talks with Chinese President Xi Jinping, where North Korea is expected to be a key point of their discussions.

Evan Medeiros, senior director for Asian affairs at the White House's National Security Council, told reporters Friday that Obama and Xi are expected to discuss the "most effective ways to use a mix of diplomacy and sanctions" to get North Korea to give up its nuclear program.

Beijing is the first leg of Obama's three-nation trip to Asia that will also take him to Myanmar for annual regional summits led by the Association of Southeast Asian Nations, and then to Brisbane of Australia for a summit of the Group of 20 major economies. (Yonhap)