WASHINGTON (Reuters) - The United States called on North Korea on Monday to immediately release on humanitarian grounds a U.S. citizen accused of trying to topple the reclusive state's government.

Korean-American Kenneth Bae, 44, was in a group of five tourists who visited the northeastern city of Rajin on a five-day trip last November and has been held by North Korean police since then.

"We call on the DPRK to release Kenneth Bae immediately on humanitarian grounds," U.S. State Department spokesman Patrick Ventrell told reporters at his daily briefing.

KCNA, the North's official news agency, on Saturday said Bae had "admitted that he committed crimes aimed to topple the DPRK with hostility toward it," using the North's official title of the Democratic People's Republic of Korea.

"His crimes were proved by evidence," it said, adding he would soon be taken to the Supreme Court to face judgment.

The move comes amid a long-simmering diplomatic standoff between Pyongyang and Washington over the North's nuclear programs.

Tensions between North Korea and South Korea and its ally the United States have risen in recent weeks since the United Nations tightened sanctions after North Korea's third nuclear weapon test in February.

The fresh sanctions led Pyongyang to threaten nuclear strikes against South Korea and the United States.

A number of U.S. citizens of Korean descent have run into trouble in North Korea over the years, and Pyongyang has tried to use their detention to extract visits by high-profile American figures, most notably former President Bill Clinton.

A U.S. official who spoke on condition of anonymity said Bae had entered the North with a valid visa and that the United States did not want his case exploited for political gain.

"In the past there have been many ... instances where American citizens are used as political bargaining chips and our concern is that this individual not be used in that manner," the U.S. official said.

(Reporting by Arshad Mohammed; Editing by Jackie Frank)