The United States on Monday urged North Korea to free a US citizen on "humanitarian grounds" as it sought to pin down the charges against him ahead of a planned trial by Pyongyang.

The North's official Korean Central News Agency (KCNA) said Saturday Pae Jun-Ho, known in the US as Kenneth Bae, had admitted trying to overthrow the communist regime in North Korea or the DPRK, and would face "judgment" soon.

"The welfare of US citizens is a critical and top priority for this department. We call on the DPRK to release Kenneth Bae immediately on humanitarian grounds," deputy acting State Department spokesman Patrick Ventrell said.

Washington was coordinating with Sweden, which represents US interests in North Korea, in the absence of US diplomatic relations with Pyongyang.

Bae, who is believed to be a Korean-American tour operator, was arrested in November as he entered the northeastern port city of Rason.

US officials pointed out he had entered the country on a valid visa, and admitted to concerns 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.

KCNA said Saturday that a "preliminary inquiry" had been completed.

"He admitted that he committed crimes aimed to topple the DPRK (Democratic People's Republic of Korea) with hostility toward it. His crimes were proved by evidence," it said.

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 labor 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.