MANILA (Reuters) - Philippine troops have fought one of their toughest clashes against militants loyal to Islamic State in Iraq and Syria (ISIS) in a southern town, and three soldiers were killed and 52 wounded, many by rebel bombs as they pushed forward, an officer said on Friday (Sept 1).

The Islamists shocked the country by seizing large parts of Marawi town in May. After more than 100 days of fighting, pockets of fighters remain dug in in the ruins.

The army made its push on Thursday, the eve of the Muslim holiday of Eid al-Adha, and seized a bridge in what military spokesman Brigadier General Restituto Padilla described as some of the toughest fighting yet.

At least five militants were killed, he said. "We are working to clear the remaining areas where the enemy is holding out," Padilla said in a statement.

"Following a short pause early today, to give due respect to the solemnity and significance of this day, the operations will continue without any let up," he said, referring to the Muslim holiday.

The military has expressed confidence the end is in sight for what has been its biggest security crisis in years, which started in May, but the latest casualties underscore the difficulty that they still face in the battlefield.

In all, 620 militants, 45 civilians and 136 soldiers and policemen have been killed in the fighting that has displaced hundreds of thousands of people and raised fears about Islamic State establishing a foothold in Southeast Asia.

The military has missed repeated targets and deadlines to crush the rebels in Marawi, a largely Muslim town on the southern island of Mindanao, raising questions about whether it can contain a wider rebellion.

President Rodrigo Duterte, who placed all of Mindanao under martial law until the end of the year after the militants occupied Marawi, has urged lawmakers to approve funds to beef up the army by 20,000 troops.

On Friday, Duterte said he saw no reason to lift martial law in Mindanao, citing violence in other parts of the island.

"The way it looks, there seems to be some spillover," he said, without elaborating.

Muslim rebels in the south of the predominately Christian Philippines have for generations battled for greater autonomy but in recent years hopes for peace were raised with several factions engaged in talks.

But the Marawi fighting has dimmed those hopes.