North Korea has sentenced Kenneth Bae to 15 years of compulsory labour in spite of US calls for his release on “humanitarian grounds”.

The official Korean Central News Agency said Bae, a US citizen of Korean descent, was tried on April 30 for unspecified offences against the state, including allegedly plotting to overthrow the regime, which Pyongyang says he has admitted to. Thursday’s announcement said he had been found guilty of these “crimes against the country”, reports the Financial Times.

Bae, also known as Pae Jun-Ho, was arrested in November last year after entering North Korea with a group of tourists and had been held since then.

The tough sentence comes in spite of pleas for fair treatment made by US politician Bill Richardson, who visited Pyongyang in January with Google chairman Eric Schmidt.

Washington has also urged Pyongyang to immediately free Bae. “The welfare of US citizens is a critical and top priority… We call on the DPRK to release Kenneth Bae immediately on humanitarian grounds,” US State Department spokesman Patrick Ventrell said.

The detention of Bae has prompted speculation that North Korea could use him to obtain concessions from the US amid tensions on the Korean peninsula after Pyongyang issued a series of warnings of an imminent nuclear war.