WASHINGTON, March 16 (Reuters) - The U.S. Congress should not accept a pending free trade deal with Colombia even if the pact's labor provisions are strengthened because of the country's "atrocious" record of violence against unionists, the largest U.S. labor federation said on Friday.

"No renegotiation of the U.S.-Colombia (free trade agreement) would adequately address the violence confronting trade unionists in that country," said the American Federation of Labor and Congress of Industrial Organizations, or AFL-CIO, in a letter sent this week to the Senate and U.S. House of Representatives.

The group says President Alvaro Uribe, a U.S. ally in Latin America, has not done enough to stamp out violence that killed around 200 trade unionists since talks on the trade deal began in 2004.

More than 4,000 Colombian union leaders have been assassinated since 1986, according to the U.S. State Department, the world's highest such murder rate.

"Colombia's atrocious human right record sets it apart from Peru and Panama," the AFL-CIO letter said.

The letter also comes just days after President George W. Bush paid Uribe a visit during a Latin American tour. In Bogota, Bush promised more trade and greater aid for Colombia.

The AFL-CIO has criticized pending Latin American trade pacts in the past, but was unusually harsh in describing the Colombia deal.

"It's not fixable at all," said Thea Lee, AFL-CIO policy director.

The AFL-CIO still advocates changes to the Peru and Panama deals, which Lee saw as "theoretically fixable."

Pro-union Democrats in Congress have urged the Bush administration to weave stronger rules on labor and the environment into pending trade deals.

Some warn they'll reject the deals without enforceable commitments to comply with international labor standards, like the freedom to organize and collective bargaining.

The administration has been holding talks with leading Democrats on the issue.

The three-page letter from the AFL-CIO comes as the Bush administration is already negotiating with Congress over trade promotion authority.

Officials are trying to lay conditions for a renewal of the president's fast track authority, which allows negotiators to broker trade deals that are voted on in Congress without changes. Fast track expires in June.

AlertNet news is provided by