UN refugee agency staff have been put on alert in Malaysia to protect refugee and asylum seekers in case they are affected by an impending government crackdown on illegal immigrants, the agency said on Tuesday.

"We are concerned that thousands of refugees and asylum seekers in Malaysia could be at risk of arrest, detention or deportation during the crackdown, which will be backed up by a half-million strong civil volunteer force," said Ron Redmond, a spokesman for the UNHCR.

Malaysia announced earlier this month that it will round up and deport mainly Indonesian illegal immigrants from March 1, marking the end of an amnesty which has twice been extended at Indonesia's request.

The office of the UN High Commissioner for Refugees said it had received assurances "from the highest levels of the Malaysian government" that about 47,000 refugees or asylum seekers it was dealing with would not be affected.

They include ethnic groups from Burma and people who have fled the internal conflict in Indonesia's Aceh province.

The UNHCR said it had set up "informal" arrangements with police and civil volunteer force, including an operation room they can call to check if those caught in the sweep are entitled to protection.

Monitoring has also been stepped up, as well as registration of hundreds of refugees in more isolated areas, it added.