Warplanes and drones circled over the besieged village of Baghouz. Explosions and fires illuminated columns of smoke over Baghouz as it came under intense artillery and rocket fire. On several occasions the village was struck with apparent cluster fire.
After sunset, amid thuds of artillery a large column of smoke rose up, illuminated by explosions, apparently after a strike by cluster fire.
"The fighting is direct and intense," Bali said, adding that so far there have been no casualties among SDF fighters.
Clashes had already broken out earlier in the day, with IS snipers targeting SDF positions and prompting a rattle of gunfire in response. Associated Press reporters witnessed SDF fighters take cover in a damaged building and fire back at the extremists. They packed high-caliber bullets into munitions belts and headed to the front line.
The assault was renewed days after thousands of people left the tiny village held by IS on the banks of the Euphrates River near the Iraqi border area.
Some 3,000 to 4,000 women and children are believed to still be in the pocket, along with around 500 IS fighters, said an SDF commander on the outskirts of Baghouz, who identified himself as Ali Sheikh.
Shiekh said that to minimize casualties the SDF will advance on the ground, with the U.S.-led coalition providing air support. He said IS fighters have been surrendering themselves daily and that no SDF prisoners remained after the extremists freed them over the past weeks.
The battle to completely retake the pocket is likely to take three days, an SDF official, Aras Orkesh, said earlier. He said about 2,500 SDF fighters were prepared for the assault.
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.
The SDF has been marching toward Baghouz since September. But the operation has been slowed by the unexpectedly large number of civilians who had been holed up in the pocket, most of them families of IS members who have fled with the group as its once extensive territory shrank.
Over the past few weeks, more than 10,000 civilians have been evacuated from Baghouz, streaming out daily through a corridor arranged by the two sides. But since Friday, only a small group has left.
"Our forces are ready now to start and finish what is left in ISIS's hand" Bali, the SDF spokesman, tweeted Sunday, using another acronym to refer to IS.
A U.S. senior defense 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 official who could not be identified by name under Pentagon ground rules said that nearly all of the 20,000 people, including women and children who left the area held by IS recently, are seen as IS followers or adherents.
The fight against IS has taken place amid Syria's nearly 8-year-old civil war.
In southern Syria, dozens of people demonstrated Sunday in the city of Daraa to protest the construction of a statue of late Syrian President Hafez Assad, days before the country marks the eighth anniversary of the country's crisis, Syrian opposition activists said.
Daraa is the city where the Arab Spring-inspired uprising began with peaceful protests in March 2011 but escalated into an armed rebellion that has killed more than 400,000 people. Last year, Syrian government forces captured all parts of southern Syria, including parts of Daraa city where tensions still exist.
The late Assad is the father of President Bashar Assad whose forces have made major gains over the past few years in the war with backing from his strong allies, Russia and Iran.
The Britain-based Syrian Observatory for Human Rights, a war monitor, said the protest occurred in Daraa's center with protesters chanting "Long live Syria" and "Down with Bashar Assad."
The Observatory's chief Rami Abdurrahman said security forces did not interfere with the protest.
Other opposition activist collectives, including Sham Network, also reported the protest.
Also Sunday, Assad met with China's assistant foreign minister, Chen Xiaodong, telling him that the war against Syria is now taking a new form which is mostly economic warfare.
The U.S. and the European Union have imposed sanctions on Syrian businessmen and entities they consider close to the government.
Associated Press writer Bassem Mroue in Beirut contributed to this report.