GUATEMALA CITY (Reuters) - The rape of a teen-age girl on a Guatemalan coffee farm is raising doubts about the Central American country's ability to clean up its labor record and win U.S congressional support for a free trade agreement.

The 15-year-old told police she was washing clothes with her younger brother on the Maria Lourdes farm in southern Guatemala on July 6 when they were attacked by masked gunmen, who beat them both and raped her.

The victim was discovered by farm employees tied up near coffee trees on the farm, which workers say belongs to the family of President Oscar Berger's sister in law.

The 15-year-old's father is one of 47 farm workers fired in 1992 when they formed a trade union to pressure employers to pay them the minimum wage. They went to court to get their jobs back and compensation, and they have won some interim rulings which the farm owners have not complied with.

Amnesty International says the rape was a warning to dissuade union members from pushing for compensation from the farm owners and U.S. Rep. Sander Levin, a Democrat from Michigan, called the anti-union atmosphere in Guatemala unacceptable.

"I find it so beyond belief and acceptance that rape is used as a tactic to fight unions," Levin said in a telephone interview from the United States.

Guatemala is one of five Central American nations which, along with the Dominican Republic, have signed a free trade pact known as CAFTA with the United States.

The deal has become bogged down in the U.S. Congress, which must approve the pact, largely because of concerns that labor rights and environmental protections are lax in those countries.

Democratic presidential candidate John Kerry has promised to renegotiate the trade pact if elected in November to include "strong protections" in both those areas.

CAFTA requires Central American countries to enforce their existing legislation, but critics say this is not enough.

"CAFTA cannot pass the way it is currently drafted. The failure to put internationally recognized labor rights in an enforceable way is a fatal flaw and no double talk will overcome it," Levin said.

While supporters of the pact still harbor hopes of getting it through Congress without changes, they currently do not have the votes.

"NO LAW, NO JUSTICE"

The rape case, if indeed linked to labor rights, is an example of the kind of abuse that blights the Guatemalan countryside, where poor peasants say landowners treat them like property.

"There is law for the rich, but for us, the people, there is no law and no justice," the raped girl's mother, Margarita Perez, said.

Rural workers' rights came under attack after a U.S.-backed coup in 1954 and have been weak since. Activists say Berger's ties to the landowning sector cast doubt on his commitment to resolving such disputes.

Guatemala's land registry shows Maria Lourdes was owned by a sister in law of Berger until 1997 and workers say her family still owns the farm. Farm spokesman Mario del Cid refused to say who the farm's owners are.

No arrests have been made in the rape case. A police doctor has confirmed the girl was raped. She identified her attackers as a security guard and the manager of the farm, prosecutors say.

Prosecutor Rodolfo Hernandez said initial investigations showed it was not clear whether the rape was related to labor disputes at the farm.

"Just because some trade unionists want to link this with the administration of the farm does not mean that's the way it is," Hernandez said.

Workers say they warned police in February that the farm's manager had threatened women with rape if the labor case was not dropped.

Farm spokesman del Cid said he suspected the workers were manipulating the rape case to remove the farm manager from a team negotiating a solution to the 12-year labor conflict.

The dispute has lingered for years in Guatemala's notoriously slow labor tribunals.

The farm owners have not complied with court rulings to reinstate and pay compensation to the sacked workers, most of whom continue to live in houses where they were born on the perimeter of the farm.

"DAILY RIGHTS ABUSES"

Since Berger took office in January, the government has made progress on a number of high profile rights cases dating back to Guatemala's 36-year civil war.

However, the attention brought by the Maria Lourdes case hurts government claims that progress is being made on improving treatment of Guatemala's rural poor.

"We see an incongruence; we are witnesses to daily rights abuses in the agricultural sector," said Henry Hernandez, spokesman for Guatemala's government-appointed Human Rights Ombudsman.

Guatemalan law makes it almost impossible to form trade-wide unions, in effect limiting unions to individual companies.

Violence against workers frequently goes unpunished and illegal lay-offs are common in industry and agriculture.

After world coffee prices collapsed several years ago, thousands of rural workers were laid off without severance pay from farms where they were born and raised.