By Hugh Bronstein

BOGOTA, June 14 (Reuters) - A proposed cut in U.S. aid would hobble Colombia's anti-cocaine efforts and result in tonnes more of the highly addictive drug reaching the United States, Defense Minister Juan Manuel Santos said on Thursday.

Democrats who control the U.S. Congress say Colombia has not done enough to curb human rights abuses and cut cocaine exports, which run at between 600 and 700 tonnes per year.

They propose cutting $110 million per year in aid supporting the spraying of herbicides on crops used to make cocaine, a move that Santos said would result in "a lot" more of the drug reaching U.S. streets.

"Our preliminary calculations show that we would have to reduce by one-third our coca eradication efforts, which would mean a corresponding increase in cocaine production," Santos said.

Colombia is the world's biggest producer of the drug and the United States is its biggest customer.

"My message to the U.S. Congress is please don't weaken aid at this particular moment," Santos said at a news conference announcing a 9 percent cut in coca leaf production last year versus 2005, according to United Nations estimates.

"It's like being on a diet," he said. "Losing the first kilos (pounds) is easy but last ones are difficult. That's the stage where we are now in the eradication of coca. To pull the rug out from under us would come at tremendous cost."

The Andean country is in a four-decade-old guerrilla war involving Marxist rebels and right-wing paramilitaries, both funded by the multibillion-dollar cocaine trade.

Farmers who refuse to grow coca to sell to these groups are forced at gunpoint to flee their homes. The United Nations says more than 3 million Colombians have been displaced while more than 40,000 people have been killed in the war since 1990.

President Alvaro Uribe is under fire because of a scandal linking some of his lawmaker allies to paramilitary warlords who have officially disarmed but stand accused maintaining their criminal networks from prison.

The paramilitaries are guilty of some of the worst massacres and other atrocities of the war.

The scandal has prompted some Democrats to question the success of "Plan Colombia," under which the United States has granted billions of dollars in mostly military aid since 2000. They are reviewing whether future aid should focus more on alternative programs to develop the rural economy and ween farmers off growing coca.

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