At least 18 soldiers were killed and more than 50 others wounded on Saturday in fierce fighting with the armed group Abu Sayyaf and allied fighters on a southern island in the Philippines.

It was the largest single-day combat loss by government forces this year in the restive south, where the military has long battled Muslim separatist rebels and Communist fighters.

Basilan is one of the remotest islands in southern Philippines [Al Jazeera]

Three military officials told the AP news agency the heavy daylong fighting took place on Mindanao island of Basilan. They spoke on condition of anonymity because they were not allowed to publicly discuss details.

Local media reports said about 100 Abu Sayyaf fighters clashed with troops and four soldiers had been decapitated. The evacuation of wounded soldiers was continuing late on Saturday. 

In 2015, more than 30 police commandos were killed by Muslim rebels during a government raid on mainland Mindanao. 

Some Muslim rebel groups in the area have reportedly allied themselves with Islamic State of Iraq and the Levant (ISIL) followers.

Abu Sayyaf is known to maintain a base in Basilan, as well as the neighbouring Sulu archipelago, where a former priest from Italy was released on Friday after millions of pesos in ransom was reportedly paid.

Abu Sayyaf was founded in 1991 in Basilan, about 880km south of the capital, Manila.

The United States and the Philippines have separately blacklisted Abu Sayyaf as a "terrorist" organisation for deadly bombings, extortion, kidnappings for ransom, and beheadings of locals and foreigners, including Christian missionaries in the south.

More than a decade of US-backed Philippine offensives have weakened the armed group, but it remains a key security threat.

 Breakaway Philippine rebels align with ISIL

Source: Al Jazeera and agencies