The kidnappers, like their teenaged target, were Chinese nationals, said the interior department's Juanito Victor Remulla, part of a "sophisticated" syndicate with ties to the now-banned offshore gambling sites known locally as POGOs.

Notorious as fronts for human trafficking, money laundering and fraud, POGOs were banned by President Ferdinand Marcos last year, sending those who worked for them in search of new income streams.

"We are definite that the syndicate behind the kidnapping were former POGO operators," Remulla told reporters, adding those involved had lost a lucrative living when the sites were shuttered.

The kidnappers tried and failed to obtain a ransom -- at one point sending the parents a video of the victim's finger being severed -- before they were tracked down on Tuesday and pursued by police who homed in on their cellphone signal.

"The choice was pursuing the vehicle or securing the child. Obviously, the (police) prioritised the child," Remulla said. A manhunt remains underway.

The boy's driver, who had picked him up outside an exclusive private school days earlier, was found murdered inside another vehicle in Bulacan province north of Manila.

"These (cases) arose in January after all POGOs were closed, they got into kidnapping," Remulla said, without providing statistics.

AFP is aware of at least two other kidnapping cases involving Chinese nationals living in the Philippines this year.

While describing the incident as "Chinese against Chinese" crime, Remulla said disaffected former Filipino police or soldiers were likely used as footsoldiers in some cases.

Gilberto Cruz, chief of the Philippines' anti-organised crime commission, told AFP that government figures showed there were still about 11,000 Chinese nationals in the country after the gambling sites they worked for were shuttered.

"Some have turned to other crimes, but we can't provide numbers as of now," he said, before adding that some had likely ventured into "kidnapping operations".

At a press conference on Wednesday, the immigration department said about 300 foreign nationals linked to POGOs were being held at a detention facility built for 100 while awaiting deportation.

In a separate statement, the department said 98 Chinese nationals had been repatriated to China aboard a chartered Philippine Airlines flight on Tuesday night.

The Chinese embassy said the joint repatriation marked "another step in the law enforcing cooperation of the two countries after the ban on POGOs".