American Lori Berenson (31), given a 20-year jail sentence for collaborating with Marxist rebels in Peru, called the latest verdict unjust.
A civilian court convicted her in Lima late on Wednesday of being a willing collaborator with the Tupac Amaru Revolutionary Movement, although not a militant member, and said she should be jailed until November 29th, 2015, counting time served.
The New Yorker showed no emotion as the sentence was read out.
"Convicting her of crimes of terrorism in respect of collaboration against the state, the jail sentence is 20 years, to end on November 29th, 2015," the verdict of the three judges said. Peru has no jury system.
Berenson, who has already served more than 5 1/2 years after a 1996 military trial, declared the conviction "unjust". She said she was innocent and would appeal to the Supreme Court.
She is also pursuing her case with the Inter-American Court of Human Rights, the region's top rights court, in Costa Rica.
The US State Department said it sympathised with her family and hoped the Supreme Court would "look carefully" at issues raised by the defence.
Berenson, who said she was researching articles on poverty in Peru for two left-wing US journals, was arrested in November 1995 in the company of the wife of a Tupac Amaru leader whom she said she had hired as a photographer without knowing her identity.
She was convicted in January 1996 at a summary military trial with a gun to her head and sentenced by a hooded military judge to life in jail as a Tupac Amaru leader. That conviction was overruled last year and a civilian trial ordered.
Berenson, a former Massachusetts Institute of Technology student, admits to sharing some of the rebels' positions but says she never espoused violence.
However, the judges found her guilty of collaborating in a plot to attack Congress. The Tupac Amaru made headlines with a 126-day siege at the Japanese ambassador's residence in Lima that ended in a 1997 raid that rescued 71 hostages but killed all 14 rebels, a hostage and two police commandos.