(Recasts; adds reaction from Colombian government, paragraph 10)

By Patrick Markey

BOGOTA, April 19 (Reuters) - A leading U.S. senator has frozen $55.2 million in military aid to Colombia over concerns about human rights and accusations that Colombia's army chief colluded with illegal paramilitaries.

The move by Sen. Patrick Leahy, a Vermont Democrat, came as some members of his party push for a tougher line with Colombia, which receives billions of dollars in U.S. aid to fight left-wing rebels and the illicit drugs trade.

Colombian President Alvaro Uribe is under scrutiny as he fends off a scandal linking several of his lawmaker supporters to the militias, who are accused of drug-trafficking and massacres during their dirty war with left-wing guerrillas.

Leahy froze the aid in his role as chairman of the Senate's state and foreign operations subcommittee, which oversees foreign programs and assistance. Democrats have controlled both houses of Congress since January.

"He has put a hold on the release of the final component from fiscal 2006 to facilitate further discussion with the State Department," said David Carle, a Leahy spokesman.

The frozen aid is part of assistance to the armed forces that needs approval by Congress, which monitors improvement of Colombia's human rights situation. The Bush administration recently certified that Colombia had made progress on rights.

But Carle said the senator wanted to discuss concerns over abuses and a Los Angeles Times story last month citing a CIA report on Colombian army commander Gen. Mario Montoya working with the right-wing militias to fight against the rebels.

"Until those discussions are held, he will continue his hold on the release of this funds," Carle said.

Leahy has previously frozen funds and released them after subsequent talks.

"We are confident that investigations will lead to the release of these funds as I have every confidence Gen. Montoya has always acted in a transparent and upright manner," Foreign Minister Fernando Araujo told local Caracol radio.

REGIONAL ALLY

Uribe has been one of the Bush administration's staunchest allies in Latin America. Under the Plan Colombia project, the White House has provided Bogota with nearly $4 billion in mostly military and counter-narcotics aid since 2000.

Washington wants to extend around $600 million a year in assistance, but some U.S. Democrats are demanding Colombia make more progress to combat paramilitary influence and defend rights activists.

Rights groups have long charged militia commanders worked in league with the armed forces to assassinate suspects they believed were linked to the guerrillas, including rights workers and labor union leaders.

"The U.S. Congress should maintain a hold on military assistance to Colombia until alleged links between paramilitary groups and state officials are thoroughly investigated," U.S.-based rights groups, including Amnesty International and Human Rights Watch said in a joint statement.

Uribe is popular among Colombians after his U.S.-backed security crackdown reduced violence by driving the rebels back into the jungles. He has negotiated the disarmament of 31,000 paramilitaries who once fought the guerrillas.

But rights groups say paramilitary commanders jailed under the peace deal have continued to run their drug-smuggling and criminal networks and are threatening victims of militia violence who are now seeking justice.

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