SEOUL (AFP) - The US on Wednesday accused North Korea of jailing an American student for political reasons and called for his immediate release after Pyongyang sentenced him to 15 years' hard labour for stealing a propaganda sign from a hotel.

Otto Warmbier, a 21-year-old student from the University of Virginia, was convicted of subversive activities and sentenced by North Korea's Supreme Court, the North's official KCNA news agency said.

White House spokesman Josh Earnest charged that North Korea was using US citizens as "pawns to pursue a political agenda" and urged it to free him.

"We strongly encourage the North Korean government to pardon him and grant him special amnesty and immediate release," said Earnest.

"The allegations for which this individual was arrested and imprisoned would not give rise to arrest or imprisonment in the United States or in just about any other country in the world."

Observers said the harsh sentence was likely a reflection of soaring military tensions on the divided Korean peninsula following the North's nuclear test in January and long-range rocket launch a month later.

The United States took a leading role in securing the resulting sanctions that the UN Security Council imposed on the North earlier this month.

In recent weeks, Pyongyang has maintained a daily barrage of nuclear strike threats against both Seoul and Washington, ostensibly over ongoing large-scale South Korea-US military drills that the North sees as provocative rehearsals for invasion.

In announcing the jail sentence, KCNA said Warmbier had committed his offence "pursuant to the US government's hostile policy" towards North Korea.

Warmbier had initially been arrested in early January on charges of "hostile acts" against the state. KCNA said he was convicted under an article of the criminal code dealing with subversion.

"In the course of the inquiry, the accused confessed to the serious offence," it said, without elaborating.

Warmbier was arrested as he was leaving the country with a tour group. He later said he had removed a political banner from the staff-only area of the Pyongyang hotel where the group had stayed.

The sentence was handed down just hours after veteran US diplomat Bill Richardson reportedly met two diplomats from North Korea's UN office in New York to press for Warmbier's release.

In the past, North Korea has used the detention of US citizens to obtain high-profile visits from the likes of former US presidents Jimmy Carter and Bill Clinton in order to secure their release.

Richardson has travelled to North Korea several times over the years on diplomatic missions that have included negotiating the freedom of arrested Americans.

The United States has no diplomatic or consular relations with the North. The Swedish embassy in Pyongyang provides limited consular services to US citizens detained there.

Warmbier is one of three North Americans currently detained in North Korea, which recently sentenced a 60-year-old Canadian pastor to life imprisonment with hard labour on sedition charges.

The US State Department "strongly recommends against all travel" to North Korea and specifically warns of the risk of arrest.

Human Rights Watch said the severe sentence was shocking given that Warmbier's alleged offence amounted to little more than a "college-style prank".

"Pyongyang should recognise this student's self-admitted mistake as a misdemeanour ... release him on humanitarian grounds, and send him home," said Phil Robertson, deputy director of the rights watchdog's Asia Division.

Detained foreigners are often required to make a public, officially-scripted acknowledgement of wrongdoing, and Warmbier was paraded in front of reporters and diplomats in Pyongyang last month.

Footage of the carefully orchestrated event showed a sobbing Warmbier pleading to be released and saying he had made "the worst mistake of my life".

Warmbier said he had been tasked with stealing the banner by a member of the Friendship United Methodist Church in Wyoming, Ohio, who wanted it "as a trophy" and offered him a used car worth $10,000 if he succeeded.

Political slogans extolling the achievements of the country and its leaders and encouraging citizens to work harder and demonstrate their loyalty are pervasive in North Korea.

They can be seen on the streets and in nearly every public building, as well as every work unit.