BEIJING (Reuters) - China is concerned over the Philippines’ detention of more than a thousand Chinese nationals during a crackdown on online gambling, the Foreign Ministry said on Monday, in what could be a strain for ties that have warmed in recent months.

The Philippines immigration bureau last week detained about 1,200 Chinese people working in call center-like facilities suspected of running online gambling operations out of a former U.S. air base, Clark Field.

The detentions marked the government’s biggest such round-up of Chinese nationals in the country, many thought to be working there illegally.

Chinese Foreign Ministry spokesman Geng Shuang said China had requested that the Philippines “appropriately make arrangements” for those who had been detained and quickly release individuals who have “legal identification”.

“At the same time, China expresses concern over the large number of Chinese citizens detained by the Philippines,” Geng told reporters at a regular press briefing, adding that China always reminds its citizens who travel abroad to abide by local laws.

It is unclear if the Chinese nationals were being held for gambling offences or for visa and immigration violations, but about 900 were brought to the immigration detention center in Manila, according to a Philippine police report.

Online gambling is not illegal in the Philippines, but it is subject to permitting and zoning regulations.

China has expressed support for the Philippines’ other law enforcement efforts, including President Rodrigo Duterte’s anti-narcotics campaign, which has sparked criticism from Western countries over fears of extrajudicial killings.

In contrast with the previous Philippine government’s adversarial relationship with Beijing, Duterte has made efforts to improve ties with China, which have been plagued by territorial disputes in the South China Sea.

Prior to leaving for an Asia-Pacific summit in Lima this month, Duterte said if Russia and China decided to create a “new order” in the world, he would be the first to join.