Philippine police are investigating the killing of an American marine biologist who was shot dead by three masked men who forced their way into his home in the central Philippines over the weekend, officials said Tuesday.

Police said Kent Carpenter, 73, was at his house in the coastal town of Sibulan in Negros Oriental province with his Filipina companion on Sunday night when three men entered the home.

According to police, citing Carpenter's companion, one of the intruders pulled out a gun and shot Carpenter in the head, killing him instantly. The attackers then fled with a laptop, an unspecified amount of cash and a backpack, national police spokesperson Col. Allen Rae Co told reporters.

Regional police spokesperson Lt. Col. Joem Malong said Carpenter's companion sustained unspecified injuries during the incident and was receiving treatment.

Investigators are working to determine the motive behind the killing and identify the suspects. Malong said Carpenter, a marine biologist, had served as a lecturer at Silliman University in Dumaguete City, Negros Oriental. He added that the U.S. Embassy in Manila had been informed of the incident.

The U.S. Embassy did not immediately respond to a request for comment from The Associated Press.

"We assure the victim's family, the community and our foreign visitors that this case is being treated with utmost urgency and no effort will be spared until justice is served," regional police director Brig. Gen. Romano Cardiño said.

Several environmental and biodiversity organisations in the Philippines expressed grief over Carpenter's death.

Silliman University described Carpenter as an outstanding scientist who had collaborated with the institution on marine research and studies since 1976.

"Dr. Carpenter made groundbreaking contributions that transformed global understanding of Philippine marine biodiversity," the university said.