ZAMBOANGA CITY - Islamist extremist group Abu Sayyaf on Monday (June 13) executed the second of three foreigners it took from a high-end resort in the strife-torn southern Philippine island group of Mindanao, amid a military rescue operation involving up to eight battalions.

The group carried out its threat to behead Mr Robert Hall, 51, a Canadian, at 3pm on Monday in Jolo island, Sulu province, some 1,000km south of the capital Manila, the Philippine Daily Inquirer reported, citing a spokesman named Abu Raami.

The group had said it would kill Mr Hall if its demand for 300 million pesos (S$9 million) was not met by Monday.

A separate military source in Jolo confirmed with The Straits Times Mr Hall had been executed. Citing raw intelligence on the ground, Major Filemon Tan, spokesman of the Western Mindanao Command, said Mr Hall was killed at 3.15pm.

But Brigadier-General Restituto Padilla, the Philippine military spokesman, said: "I cannot confirm or deny it. We don't have reports yet coming from our units on the ground.”

Canadian Prime Minister Justin Trudeau said his government has “compelling reason to believe” that Mr Hall has been killed, Agence France-Presse reported.

“It is with deep sadness that I have reason to believe that a Canadian citizen, Robert Hall, held hostage in the Philippines since September 21, 2015, has been killed by his captors,” Trudeau said in a statement.

“While Canadian officials are working closely with authorities in the Philippines to formally confirm Mr Hall’s death, we have compelling reason to believe that reports to this effect are, unfortunately, true,” Trudeau said.

“The vicious and brutal actions of the hostage-takers have led to a needless death. Canada holds the terrorist group who took him hostage fully responsible for this cold-blooded and senseless murder.”

Mr Hall was taken on Sept 21 last year (2015) by the Abu Sayyaf from the Holiday Oceanview Samal resort on Samal island in Davao together with another Canadian, Mr John Ridsdel, Norwegian resort manager Kjartan Sekkingstad, 57, and a Filipina, Ms Maritess Flor, 41.

Mr Ridsdel, 68, a former mining executive and journalist, was beheaded on April 25 after a 300 million pesos placed on his life was not paid.

The Inquirer reported that Mr Hall, just hours before his scheduled execution, had appealed to the Philippine government “to get us all out here”.

“My condition is pretty bad. We have been starved, our sleep is deprived and they threaten to beat me,” Mr Hall told the Inquirer by phone.

Last week, the Abu Sayyaf freed four Malaysian sailors more than two months after they were abducted off Borneo island.

Fourteen Indonesians kidnapped by the group were released last month.

The Abu Sayyaf, whose chieftain Isnilon Hapilon has been recognised by the Islamic State in Iraq and Syria as a council leader, is known for extortion, kidnappings, beheadings and bombings.

It was formed by disgruntled Moro Islamic fighters in 1991, with Al-Qaeda funding.

Philippine President Benigno Aquino had ordered security forces to “apply the full force of the law… to neutralise these lawless elements, and bring these criminals to justice” following Mr Ridsdel’s murder.

Efforts to hunt down heavily armed, well-stocked and mobile groups of Abu Sayyaf bandits with a network of checkpoints and battalions of soldiers have proven to be ineffective – and even fatal for both troops and hostages - in the past.

On April 9, Abu Sayyaf gunmen killed 18 Filipino soldiers searching for hostages in a day-long battle.

The only operation that led to some measure of success was in 2002, when US-backed Philippine special forces units ambushed a group of bandits holding American missionaries Martin and Gracia Burnham, and a Filipino nurse, Ms Ediborah Yap.

Mrs Burnham was rescued, but her husband and Ms Yap were killed in the ensuing crossfire.

In most cases, hostages were released after payment of millions worth of ransom. One of the Abu Sayyaf’s biggest recent windfalls is believed to have come in 2014 when it claimed to have been paid more than US$5 million (S$6.7 million) for the release of German physician Stefan Okonek, 71, and his companion, Ms Henrike Dielen, 55, abducted from aboard their yacht.