WASHINGTON (AFP) - US officials vowed Tuesday to "leave no stone unturned" to free three Americans held by North Korea, after the men urged Washington to send an envoy to negotiate their release.

As Pyongyang's minders looked on, an American reporter on an official tour was granted surprise, separate five-minute-long interviews with Kenneth Bae, Jeffrey Fowle and Matthew Miller, who provided new details about their treatment in the autocratic country.

"We are in regular contact with the families," State Department spokeswoman Jen Psaki told reporters, adding Washington had been doing "everything we can to bring these three US citizens, detained in North Korea, home".

"There is no greater priority to us than the welfare and safety of US citizens abroad. So we will leave no stone unturned in that regard," she said. But Psaki refused to outline US efforts publicly, saying the goal was bring them home safely and Washington did not want to jeopardize any diplomacy.

She also said she had no information about media reports that US officials had recently travelled to the country. She also would not discuss whether Washington was prepared to send a high-level envoy to Pyongyang as it has in past cases, when former president Bill Clinton and ex-governor Bill Richardson successfully won the release of detained Americans.

The US special envoy for North Korean human rights issues, Robert King, has twice tried to visit to secure Bae's release, only for Pyongyang to cancel at the last minute.

Bae's family has voiced fears he is being used as a bargaining chip by North Korea after his arrest in November 2012. He was later sentenced to 15 years of hard labour on charges of seeking to topple the North Korean government.

His sister Terri Chung said it was evident from the CNN video that her brother, who wore a button-down shirt and sat on a chair during the interview, was under a lot of stress.

Miller was arrested in April after he apparently ripped up his visa at immigration and demanded asylum.

Fowle entered the North on April 29 and was detained after reportedly leaving a Bible at a hotel. North Korea said in July it would put Miller and Fowle on trial on unspecified charges related to "perpetrating hostile acts."

Washington has no diplomatic ties with North Korea, and the Swedish embassy acts as a go-between in such consular cases. Swedish officials last visited Bae on August 11, and saw Fowle and Miller in late June.