Malaysian police said Tuesday they had rescued 21 Ugandan women from forced prostitution, breaking up what they called the country's first large-scale such ring involving African victims.

The Federal Criminal Investigation Department said in a statement that police freed the women on Friday from four flats in an apartment building on the outskirts of the capital Kuala Lumpur.

The women, aged from 19 to 42, were lured to Malaysia with promises of jobs as maids in homes and hotels but instead were forced into prostitution to pay off their $7,000 travel fees.

The women "were exploited as sex slaves," the statement said. "The victims were threatened physically and mentally if they ran away and report to authorities."

It added that all of them were brought from Uganda to Malaysia via China.

If they refused sex, they were beaten and raped, a police official in the department said.

"This is the first time we have such a large number of Africans," said the official, who declined to give his name as he was not authorised to speak publicly about the case.

He added there had been previous cases involving individual African prostitutes but not a syndicate.

Police have also arrested two Ugandan woman believed to be pimps and a Ugandan man believed to be a customer, the department statement said.

The official said police were still investigating how long the ring had been in operation and whether there were more victims.

The penalty for human trafficking in Malaysia is up to 15 years in jail.

Although prostitution is illegal in Muslim-majority Malaysia, it is prevalent especially in urban centres.