SEOUL • North Korea sentenced a US college student to 15 years hard labour for what it called hostile acts against the country, further chilling relations with the United States as the Obama administration seeks to ratchet up pressure on the regime over its nuclear arms programme.

North Korea's Supreme Court yesterday imposed the sentence on Otto Frederick Warmbier, a 21-year-old student at the University of Virginia, the country's official Korean Central News Agency (KCNA) said.

Warmbier entered North Korea as a tourist in late December and took down a propaganda sign in Pyongyang so he could exchange it for money outside the country, KCNA reported in late February.

KCNA said Warmbier had committed his offence "pursuant to the US government's hostile policy" towards North Korea. He was convicted under an article of the criminal code dealing with subversion, KCNA said.

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

Warmbier was arrested in January as he was leaving the country with a tour group.

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

The sentencing comes as the US is conducting annual military exercises with South Korea, which North Korea has denounced as a dress rehearsal for war.

Early this month, the UN Security Council tightened sanctions on North Korea, imposing a ban on exports of certain minerals - a key source of hard currency for the Kim Jong Un regime - over its fourth nuclear test in January and a long- range rocket launch weeks later.

North Korea has detained and convicted US nationals as a way to draw prominent American figures such as former US president Jimmy Carter into Pyongyang as mediators to open negotiations with Washington.

BLOOMBERG