Armed gunmen raided the centre on Wednesday, according to Pierre Legros, the regional co-ordinator of Afesip, the group which runs the shelter.

The attackers drove off with many of the women at the shelter, who had only recently been rescued from a massage parlour, he told the BBC News website.

Afesip is now calling on the Cambodian government to intervene.

Staff have received death threats and we have to think of our own safety and that of our kids Pierre Legros, Afesip

In the past few weeks, Afesip became aware of the existence of a particular massage parlour in Phnom Penh which employed very young girls, Mr Legros said.

The group informed the police, who raided the establishment on Tuesday and brought the women to the Afesip shelter.

But just a day later, more than 30 people - some armed with handguns - surrounded the shelter, threatened staff and removed the women, many of whom were young girls.

It remains unclear where they were taken, but according to information obtained by Mr Legros, some are already back in the massage parlour.

He added that he was very concerned for their welfare, and for the welfare of the others who were still unaccounted for.

Because of the dangerous situation, Afesip has been forced to cut back its work at the shelter, at least until early next week, Mr Legros said.

"We will definitely continue doing some things, but the staff have received death threats and we have to think of our own safety and that of our kids," he said.

"We want the Cambodian government to protect us," he added.

Somaly Mam, the president of Afesip, told the Associated Press news agency that without government intervention: "We don't know how we can continue to operate".