Tjostolv Moland, one of two Norwegian citizens imprisoned in Congo since 2009 on murder charges, has died in prison.

British-Norwegian citizen Joshua French, 31, who was imprisoned alongside the 32-year-old, is upset but in good health, lawyer Hans Marius Graasvold said. The cause of Moland's death was not immediately clear.

The two former Norwegian soldiers were convicted of espionage and the murder of a local driver and sentenced to death. They have consistently denied the charges against them, and the Norwegian authorities have tried to have them transferred to serve their sentences in Norway.

Norway's foreign minister, Espen Barth Eide, said Moland was found dead in his cell on Sunday morning and that his family had been informed.

"More than anything our thoughts now are with them in this very difficult situation," Eide said, adding that the government had been concerned about French and Moland for a long time.

Norway and Britain have worked together to find a solution for French and Moland, and Eide said he has kept Britain's foreign secretary, William Hague, informed.

One of two Norwegian men jailed in the Democratic Republic of Congo since 2009 on charges of murder, espionage and conspiracy was found dead in his cell on Sunday, Norwegian and Congolese authorities said.

Tjostolv Moland, 32, had been detained along with the British-Norwegian citizen Joshua French since the 2009 murder of their driver in Congo's jungle. The pair denied the charges. Norway has maintained that the men, both former military servicemen, were not spies.

The Norwegian foreign minister, Espen Barth Eide, would not speculate about the cause of death but said Norway would use "full pressure" to seek clarification. Congolese authorities "bear full responsibility" for providing answers, he added.

Lambert Mende, Congo's information minister, said the Congolese government was investigating Moland's death.

"We're trying to determine whether it was suicide or homicide. It looks like suicide but we're not sure," he told Reuters, without giving further details.

A Congolese court at one stage handed down the death penalty and ordered the Norwegian government to pay more than $500m in damages. The death penalty was later overturned by Congo's military high court.

"We have been working with Congolese authorities through various channels over the years, some public, others not, without any success. I can honestly say that no case in the ministry has demanded more of our resources and attention," Eide told a news conference in Norway.

"Our main focus now is to get French home. ... That's more important than ever," he said.

Moland's death overshadowed even the upcoming elections in Norway's media.

Moland's lawyer, Hans Marius Graasvold, told the daily Aftenposten that Moland had been ill several times since 2009 and has been treated for malaria and psychosis.

He said Moland and French shared a cell for most of the time but would not say if French found the body or if they were in the same cell at the time of death.