North Korea is to put a detained US citizen on trial on September 14, state media has said, less than a week after Matthew Miller made a highly unusual televised plea for help from Washington.

Miller, one of three Americans being held in North Korea, was arrested in April after Pyongyang said he ripped up his visa on arrival at immigration and demanded asylum.

"The Supreme Court of the Democratic People's Republic of Korea (North Korea) decided to hold on September 14 a court trial on American Matthew Todd Miller, now in custody according to the indictment of a relevant institution," the official news agency KCNA said.

The statement offered no further details, the AFP news agency reported.

North Korea said in June it would put Miller and another detained US citizen, Jeffrey Fowle, on trial on unspecified charges related to "perpetrating hostile acts".

On September 1, Miller - along with Fowle and the third US citizen being held in North Korea, Kenneth Bae - pleaded for their freedom in an interview with CNN.

They urged Washington to send an envoy to the isolated state to negotiate their release.

"My situation is very urgent," Miller said during the interview.

"I think this interview is my final chance to push the American government into helping me," he added.

US officials vowed after the interviews were aired that they would "leave no stone unturned" in their efforts to free the three men.

But State Department spokeswoman Jen Psaki refused to outline US efforts publicly, saying Washington did not want to jeopardise any diplomacy.

She 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.

"We continue to work actively to secure these three US citizens' release," she said.

The State Department said there was no update to Psaki's earlier remarks after the North's announcement Sunday.

'Freedom' of religion

Fowle entered the North on April 29 and was detained after reportedly leaving a Bible at a hotel.

Bae was arrested in November 2012 and later sentenced to 15 years of hard labour on charges of seeking to topple the North Korean government.

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.

The trial date for Miller, 26, has been set as the North launches a diplomatic offensive by sending senior diplomats on rare trips to Europe - and, possibly, to the US.

Kang Sok-Ju, secretary of the central committee of the ruling Workers' Party, arrive ond Saturday for a European tour, including Germany and Italy.

Foreign Minister Ri Su-Yong reportedly plans to visit New York to attend the UN General Assembly later this month, in the first visit to the US by anyone in the role of North Korea's top diplomat in 15 years.

As part of the renewed diplomatic campaign, Pyongyang will use the US detainees as a bargaining chip to force Washington to the negotiating table, said Kim Yong-Hyun, professor of North Korean Studies in Dongguk University.

"The North is hoping that the US will send a senior-level envoy [to the North] and hoping in this process to improve ties with Washington and make progress in nuclear negotiations," Kim said.

A number of foreigners have been detained in the North for years, many for alleged involvement in religious activities.

Although religious freedom is enshrined in the North's constitution, it does not exist in practice and religious activity is severely restricted to officially-recognised groups linked to the government.