BAGHOUZ, Syria -- U.S.-backed Syrian forces pounded the last village held by the Islamic State group with artillery and heavy weapons from multiple sides Monday and made slow advances on its edges, battling militants holed up in underground tunnels.

The fighters of the Kurdish-led Syrian Democratic Forces moved toward a tent encampment in the village of Baghouz and at one point encircled a group of IS militants, killing a number of them in an hour-long battle, an SDF spokesman said.

After weeks of besieging Baghouz, the operation launched Sunday night aims at finally taking the last tiny patch of land held by the militants, a pocket on the banks of the Euphrates River in eastern Syria near the border with Iraq.

Operations against IS "will continue until we totally wipe them out," the SDF vowed in a statement Monday, saying SDF fighters had made "tangible progress" since Sunday, with 37 militants killed and five SDF fighters wounded.

Some 500 IS fighters are believed to be still in the territory, along with possibly 3,000 to 4,000 civilians, including women and children -- mainly family members who remained after thousands of civilians streamed out of Baghouz in past week during pauses in the fighting.

The fighters are heavily dug in and have laced the area with land mines and booby traps. They lashed out Monday with attempted suicide attacks -- four IS fighters assaulted SDF positions, setting off their explosive belts, though they only succeeded in damaging a minesweeper, the SDF spokesman Mustafa Bali said in a tweet.

A senior U.S. defence official said in Washington on Friday that it would not be a surprise, based on current conditions, if it took another couple of weeks to finish "mopping up" the IS enclave.

The capture of Baghouz would be a milestone in the devastating four-year campaign to defeat the group's so-called "caliphate" that once covered a vast territory straddling both Syria and Iraq.

In Monday's fighting, SDF fighters captured an arms depot and marched toward the tent settlement on the edge of Baghouz, Bali wrote. The U.S.-led coalition carried out 11 airstrikes destroying depots, fortifications and vehicles.

SDF explosives experts have begun removing mines and booby-traps "to open the way for our attacking forces," the SDF statement said.

Throughout the day, thuds of renewed artillery and heavy weapons fire could be heard. The village lies in an agricultural area squeezed between a bend in the Euphrates and the cliff of a desert plateau, and SDF forces could be seen firing at it from all sides by the evening.

"If as we advance, we notice there are civilians, we will do all we can to evacuate them from the battlefield," Bali told the AP.

The tent camp had been home for many of the thousands of civilians who had been holed up in Baghouz for weeks. On Monday, an AP team was able to see a part of the camp that had been abandoned earlier. Many tents were damaged and burned, some of them set up over empty trenches and foxholes.

The AP viewed it from a house captured by the SDF that IS appeared to have used as a clinic by IS. Medical equipment stood in place still wrapped in its plastic, as well as a microscope still in its box.

The Britain-based Syrian Observatory for Human Rights, a war monitor, said SDF fighters captured seven IS gunmen in Baghouz since Sunday.

During years of battles against IS, thousands of extremists have been taken prisoner by the SDF in Syria as well as tens of thousands of family members who were moved to tent settlements in northeastern Syria.

A senior U.S. defence official said last week that the SDF is believed to have about 5,000 IS fighters in captivity, of whom about 4,000 are Iraqis and Syrians. The other 1,000 or so are foreign fighters from dozens of other countries, the official said.

Bali said the SDF recently handed over a number of Moroccan citizens who were staying in tent settlements in northeastern Syria to their home government. Bali did not give a number, but thousands of fighters and supporters from north Africa joined IS since declaring the so-called caliphate in 2014.

Associated Press writer Bassem Mroue contributed to this report from Beirut.