Malaysia's operation to illegal migrant workers implored by Southeast Asia MPs.

The Asean Parliamentarians for Human Rights have called on Malaysia to stop the crackdown on undocumented migrant workers.

Southeast Asian lawmakers has called Malaysia to halt a crackdown on migrant workers that has seen more than 6,000 foreigners detained and sparked alarm among rights groups.

Relatively developed Malaysia is a magnet for migrant workers from across Asia in sectors ranging from construction to agriculture, but several million are believed to be undocumented.

Authorities started rounding up illegal workers after an official programme to register undocumented foreigners ended on June 30.

The latest official figures show 6,038 undocumented workers have been arrested in raids. The two largest groups represented were from Indonesia and Bangladesh, with substantial numbers also from Myanmar, the Philippines and Vietnam.

MPs from a group that brings together Southeast Asian lawmakers pushing for better human rights visited Malaysia on a fact-finding mission, and at the end of the trip made a plea for authorities to end the crackdown.

"This inhumane action must be halted," Philippine Congresswoman Emmi De Jesus told reporters in Kuala Lumpur at the end of the visit.

"Many migrants are living in constant fear. Poor treatments by law enforcement, including indefinite detention in abysmal conditions, are urgent concerns."

The group, ASEAN Parliamentarians for Human Rights, urged all Southeast Asian governments to adopt a binding regional treaty on migrant workers, aimed at protecting workers and their rights.

Leading Malaysian migrant rights group Tenaganita said it was "upset to see such large numbers of undocumented workers including refugees being handcuffed, arrested, criminalised, sent to prisons and detention centres".

The group says that many of the migrants are in reality victims of unscrupulous employers and agents who demand huge sums of money to bring them to the country, and then leave them saddled with huge debts.

Migrant workers typically do manual jobs spurned by locals. A total of 135 employers have also been detained in the crackdown, according to official figures.