Two Americans serving lengthy prison sentences in North Korea have been released by the reclusive state following a secret mission to Pyongyang by US intelligence chief James Clapper.

Kenneth Bae and Matthew Miller were the last two American nationals held by North Korea following the sudden release last month of 56-year-old Jeffrey Fowle.

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A senior State Department official said on Saturday there had been no “quid pro quo” deal in exchange for North Korea’s decision to release Bae and Miller.

US citizens Jeffrey Fowle (L), Matthew Todd Miller and Kenneth Bae (R). Photo: AAP

“The Department of State welcomes the release of US citizens Kenneth Bae and Matthew Todd Miller from the DPRK, where they have been held for two years and seven months, respectively,” a statement said.

US Director of National Intelligence Clapper travelled to North Korea and “engaged on behalf of the United States in discussions with DPRK authorities about the release of two citizens”, the statement said.

“The safety and welfare of US citizens abroad is the Department of State’s highest priority, and the United States has long called on DPRK authorities to release these individuals on humanitarian grounds. We join their families and friends in welcoming them home.”

President Barack Obama, speaking just hours before leaving for a swing through Asia, hailed the release.

“I think it is a wonderful day for them and their families and obviously we are very grateful for their safe return,” he said.

Bae, a Korean-American missionary, earlier this week marked the two-year anniversary of his detention by the reclusive state.

The sickly 42-year-old was arrested in November 2012 and later sentenced to 15 years’ hard labour.

Miller, 24, had been sentenced to six years’ hard labour by the North Korean Supreme Court following his arrest in April, after he allegedly ripped up his visa at immigration and demanded asylum.

Washington had condemned Pyongyang over the detentions, saying the Americans were held as political hostages to extract diplomatic concessions.