Iraq tipped off the Philippines about the presence of Taja Mohammad Al Jabori, who was arrested on Sunday, national police chief Ronald Dela Rosa told reporters.

However, the arrest was due to visa problems rather than any evidence of militant activity, the police chief emphasised.

"He's an illegal alien, his visa is expired so he has to be deported right away," Dela Rosa said.

"He admitted being a member of Hamas. He's a chemist and he has been responsible for improving the rocket technology of Hamas in firing their missiles from their area towards the other side, for Israel."

The suspect will be deported to Iraq.

The police chief said it was the first time Philippine authorities had dealt with an alleged member of Hamas, a group labelled a terrorist organisation by the United States, the European Union and Israel.

The Islamist movement does not recognise Israel, with which it has fought three wars, and has vied with the rival Fatah movement for control of Palestinian territory.

The handcuffed detainee did not speak while being made to stand beside the Philippine police chief at a press conference.

Dela Rosa said it was unclear at present why the alleged Hamas chemist had travelled to the Philippines.

Police said he arrived last year as Philippine troops battled militants loyal to the Islamic State group for control of the southern city of Marawi.

Al Jabori had however mostly stayed in Manila and nearby provinces and told police he had no intention of committing any terror act in the Philippines, the police chief said.