North Korea has sentenced a US citizen to 15 years of hard labour for "hostile acts" against the government, the official Korean Central News Agency has said.

Pae Jun-ho, known in the United States as Kenneth Bae, was arrested in November as he entered the northeastern port city of Rason.

"The Supreme Court sentenced him to 15 years of compulsory labour for this crime," KCNA said on Thursday.

Pyongyang has not specified the basis of the crimes allegedly committed by Pae, who is believed to be a Korean-American tour operator.

KCNA said on Saturday when announcing his trial that Pae had "admitted that he committed crimes aimed to topple the DPRK [Democratic People's Republic of Korea]".

Amnesty call

The United States called for the "immediate release" of Pae.

"We urge the DPRK authorities to grant Mr Bae amnesty," deputy acting State Department spokesman Patrick Ventrell told reporters.

"We have concerns about the lack of transparency in this case," Ventrell said, adding that Washington was still seeking details of the charges filed against Pae.

Seoul-based activist Do Hee-yoon has told the AFP news agency that he suspected Pae was arrested because he had taken photographs of emaciated children in North Korea as part of efforts to appeal for more outside aid.

US officials have pointed out Pae had entered the country on a valid visa, and admitted to concerns that he could be used as a "political bargaining" chip.

Tensions have been running high between the United States and North Korea since Pyongyang carried out a third nuclear test in February.

Several Americans have been held in North Korea in recent years.

In 2011, a US delegation led by Robert King, the US special envoy for human rights and humanitarian issues, secured the release of Eddie Jun Yong-su, a California-based businessman, who had been detained for apparent missionary activities.

In 2010, former US president Jimmy Carter won plaudits when he negotiated the release of American national Aijalon Mahli Gomes, sentenced to eight years of hard labour for illegally crossing into the North from China.

On another mercy mission a year earlier in 2009, former president Bill Clinton won the release of US television journalists Laura Ling and Euna Lee, jailed after wandering across the North Korean border with China.