THE visa status of seven foreign prostitutes is being investigated after they were tied up during a robbery at a brothel in Perth.

Three went into the brothel at Inglewood in Perth's eastern suburbs, and tied up the women, police spokesman Graham Clifford said.

"It seems they have held the place up and then left empty-handed," Sgt Clifford said.

It was unclear how the women were discovered.

Police said they were investigating the credentials of the women, believed to be Korean.