Suspected Abu Sayyaf militants have reportedly beheaded a Malaysian citizen held captive since May on the southern Philippine island of Jolo. Malaysian Prime Minister Najib Razak on Wednesday condemned the killing of businessman Bernard Then in “strongest terms.”

On Tuesday, the Philippine military reportedly said it was investigating Abu Sayyaf’s links to the execution.

"We call upon authorities to take action against those who have perpetrated this savage and barbaric act and ensure that they are brought to justice,” Najib said in a statement posted on his Facebook page. “We will also extend our full cooperation to relevant agencies and authorities in charge on the investigation of this incident.”

Malaysia’s Deputy Home Affairs Minister Nur Jazlan Mohamed called Then’s death as "violent," according to Reuters.

Abu Sayyaf, which has been listed as a terrorist group by the U.S. Department of State, was established in the early 1990s in Basilan province, about 100 miles northeast of Sulu. The extremist group, which has its stronghold in Jolo, is known for attacks, kidnappings and beheadings.

According to Reuters, Then and a Malaysian woman were abducted from a Chinese restaurant in a coastal town in the eastern Malaysian state of Sabah, on Borneo island, and taken to Jolo.

Last week, reports surfaced that Abu Sayyaf freed the woman after ransom was paid for her release.

In September, Abu Sayyaf kidnapped two Canadians, a Norwegian and a Filipino from a beach resort in the southern Philippines.