A U.S. couple detained in Uganda on charges of aggravated torture of a 10-year-old boy face an additional charge of aggravated child trafficking, which carries the death penalty if they are convicted, the state prosecutor said on Wednesday.

Nicholas Spencer and his wife, Mackenzie Leigh Mathias Spencer, both 32, have been in custody in Uganda since Dec. 9 after they were charged with aggravated torture of the child who was living in their home under foster care in the capital Kampala. They have pleaded not guilty.

The new charge sheet seen by Reuters on Wednesday showed the couple have additionally been charged with "aggravated trafficking in children." They have yet to enter a plea.

A lawyer for the couple, who were not identified, was quoted by Ugandan newspaper the Monitor as dismissing the case as a "fishing expedition" by authorities, saying they had no evidence. She was quoted as saying the new charge "doesn't make sense."

Attempts by Reuters to reach a lawyer for them via the court and the prosecutors' office were not immediately successful.

Prosecutors accuse the couple of having recruited, transported and kept the child through "abuse of position of vulnerability for purposes of exploitation," according to the charge sheet.

The new charge was read out to the couple on Tuesday when they appeared in a magistrate's court but they were not allowed to make a plea as the case can only be heard at the High Court, Jacquelyne Okui spokeswoman for the public prosecutor's office told Reuters on Wednesday.

Spencer and his wife were further detained and the date for when they will appear in the High Court to take a plea to the new charge has not yet been determined, Okui said.

"We will begin the process of committing them to the High Court but we can't say when that will be finalized so they can be produced in court," Okui said.

The first charge, aggravated torture of a child, carries a maximum sentence of life in prison.

The U.S. Embassy in Kampala did not immediately respond to a request for comment on Wednesday. It said last week it was aware of reports of the arrest and detention of two U.S. citizens in Kampala and was monitoring the situation, but had no further comment due to privacy considerations.