Immigration Commissioner Jaime Morente said the documents of the 1,240 Chinese nationals arrested Thursday were being checked based on reports they illegally entered and worked in the country.

"As of 2 am today, our immigration unit (has) counted a total of 1,240 Chinese nationals arrested in the operation," Morente said.

The immigration bureau's initial report said the foreigners were working as call centre agents for what is suspected to be an illegal gambling operator at the Fontana Leisure Park in Clark Freeport, a former US-run air base north of Manila.

But the bureau's spokeswoman Antonette B. Mangrobang said they were still awaiting the results of an investigation by the Justice Department's cybercrime unit to see if their employer was running an illegal online gambling site.

The arrested Chinese are temporarily being held under guard by immigration and police personnel in a building within the leisure park.