TEGUCIGALPA, April 24 (Reuters) - Masked gunmen killed the leader of Honduras' largest union group in an attack authorities said was a robbery attempt, but fellow unionists said she was targeted because of her job.

Rosa Fuentes, the head of the country's largest labor federation, was shot late on Wednesday after a carful of six armed men wearing ski-masks smashed into the back of her car, police spokesman Hector Mejia told Reuters.

Fuentes and the driver of the car died at the scene and another union leader died en route to the hospital.

Police said the attack was an attempted robbery by gang members. The attackers fled when another car pulled up, leaving without some $4,000 Fuentes had with her.

But fellow union members said Fuentes, whose CTH federation groups manufacturing unions with banana workers and civil servants, was killed because of her work.

"This was done by enemies of the labor movement, by hired killers," said Rigoberto Duron, No. 2 at the CTH.

Honduras is part of a regional free trade deal with the United States that has been criticized for its lax labor controls in Central American countries.

In neighboring Guatemala, five labor leaders were murdered last year. That case prompted the AFL-CIO labor federation to file a complaint with the U.S. labor department saying the government had failed to seriously investigate, violating provisions of the trade pact.

The Bush administration has been pushing Congress in recent days to approve a free trade agreement with Colombia. U.S. labor groups oppose the pact, saying Bogota has done too little to curb violence against trade union members. (Reporting by Gustavo Palencia; Writing by Mica Rosenberg, editing by Patricia Zengerle)