DHAKA (Reuters) - Bangladeshi garment factory owners use beatings, the threat of murder and sexual intimidation to stop workers from forming trade unions, a human rights group said on Thursday.

Bangladesh amended its labour law in July to boost worker rights, including the freedom to form trade unions, after a factory complex collapsed in April killing more than 1,100 garment workers, sparking debate over safety and rights.

But New York-based Human Rights Watch said it interviewed 47 workers in 21 factories in and around the capital, Dhaka, from October and it said many of the workers described abusive practices.

"The workers claimed that some managers intimidate and mistreat employees involved in setting up unions, including threatening to kill them," the rights group said in a statement.

"Some union organizers said they were beaten up, and others said they had lost their jobs or had been forced to resign. Factory owners sometimes used local gangsters to threaten or attack workers outside the workplace, including at their homes, they said."

Rock bottom wages and trade deals with Western countries have turned Bangladesh's garments sector into a $22 billion industry, that accounts for four-fifths of its exports.

In June, U.S. President Barack Obama cut off U.S. trade benefits for Bangladesh in a mostly symbolic response to conditions in its garment sector, given that clothing is not eligible for U.S. duty cuts.

Human Rights Watch said one woman said that when workers in her factory presented their union registration forms to the company owner, he threw it in a dustbin then threatened them, saying he would never allow union membership.

Two of her fellow organizers were later attacked by unidentified assailants, one with cutting shears, she said. Two weeks later, a group of men, including a known gangster and the factory owner's brother, visited her home and threatened her. She agreed to resign.

Many female workers said they received threats or insults of a sexual nature, the rights group said.

Officials at the Bangladesh Garment Manufacturers and Exporters Association were not available for comment.

A government official said he had not seen the rights group's report but said worker rights were improving.

"We are actively working to improve workers' rights after Rana Plaza incident and have made significant development," Labour Secretary Mikail Shipar said, referring to the collapse of the factory complex last year.

"After amendment of the labour law, 99 trade unions have so far been registered," he said. "Sixteen trade unions complained against their owners and we are investigating this."

Bangladesh was been under pressure to adopt a better labour law after the European Union, which gives preferential access to the country's garment industry, threatened punitive measures if it did not improve worker safety standards.

(Editing by Robert Birsel)