In this file photo, FARC rebels walk along a coca field near the village of Penas Coloradas, about 280 miles (450 kms) south of Bogota, Aug. 25, 2000

The top American anti-drug official for Latin America says the United States will use all means necessary to bring to justice 50 Colombian rebel leaders indicted in the U.S. for drug trafficking.

Assistant Secretary of State Anne Patterson told Colombia's Caracol Radio in an interview Thursday that the United States conducts interventions against drug trafficking at the invitation of the Colombian government; but, Patterson stressed that the U.S. will not take unilateral military action.

Patterson made her remarks one day after a U.S. federal grand jury indicted 50 leaders of Colombia's main rebel group, the FARC, on charges of sending $25 billion worth of cocaine into the United States and other countries. U.S. Attorney General Alberto Gonzales said the indictment was the largest of its kind ever filed in American history.

He said three of the FARC leaders are in custody in Colombia and that the U.S. is seeking their extradition. The others remain at large.

The U.S. State Department has announced more than $75 million worth of rewards for information leading to the capture of the remaining FARC leaders.

The indictment says the FARC is behind 50 percent of the world's cocaine trade and that more than 60 percent of its cocaine is sent to the United States.

Some information for this report was provided by AP .