Detainees set fire to a Malaysian immigration center and about 100 of them escaped as the blaze partially gutted the facility, officials said Tuesday.

Authorities recaptured 30 of the 109 detainees who broke out of the camp in central Negri Sembilan state late Monday, said Home Ministry official Mohamed Asri Yusof. Most were from Myanmar.

Officials were unsure how the detainees managed to start a blaze. About 1,000 other detainees remain in custody, and no injuries were immediately reported, Mohamed Asri said.

The fire and escape could refocus attention on conditions at detention centers. Activists have long said such facilities are overcrowded with detainees awaiting deportation after being arrested for illegally entering Malaysia or overstaying.

The national news agency, Bernama, quoted state deputy police chief Abdul Manan Mohamad Hassan as saying that some detainees had been at the center for up to a year.

"Investigations show that they were dissatisfied with what they perceived as cramped living conditions, the food and long detention periods," he said. Bernama said police have mounted road blocks in Negri Sembilan and were using tracker dogs and helicopters in the search.

Human rights group Amnesty International said those who escaped may have included refugees from Myanmar who are often detained with other illegal immigrants until their claims for asylum can be assessed. Malaysia doesn't recognize asylum seekers and treats them as illegal immigrants who can be arrested, caned and detained for being in the country illegally.

"We are concerned for their safety," Amnesty researcher Lance Lattig said. "We've seen how migrants at (the Negri Sembilan center) are held in conditions that fall far short of international standards."

Thousands of illegal immigrants are held at Malaysia's 13 immigration detention centers. Most of them are illegal immigrants from Indonesia who are deported within weeks.

But those with refugee claims _ many from Myanmar _ must wait for the U.N. High Commissioner for Refugees to assess their status. If the U.N. agency finds their claims to be valid, they are allowed to leave the centers and stay in Malaysia until they can be resettled to a third country, which grants them full legal status.

Malaysia legally employs about 1.8 million foreigners, mostly from poorer regional countries, on plantations, construction sites, factories and other labor-intensive industries. Authorities estimate that in addition, as many foreigners work in Malaysia illegally.