"The battle will be launched this evening and its mission will be to eliminate the last remnants of the terrorist organisation", Mustafa Bali, the head of the SDF media office, said, describing it as the "last battle".
"IS counterattack was foiled at 4 am this morning", Bali tweeted on Sunday morning.
On Saturday, Bali said he expected the battle for the last patch of territory held by ISIL to be over in days.
The Wall Street Journal reported the complete United States pullout will happen by the end of April.
But the chancellor said Germany and allies will face an uphill battle to ensure its defeat as the group has not disappeared and in her words "we remain a long way from peace in Syria".More news: Pebble Beach Pro-Am: Phil Mickelson one shot off first-round lead
Fighters supported by British and USA special forces resumed their assault on Islamic State's final redoubts yesterday after a delay of two weeks for negotiations that failed.
Earlier in the day, a war monitor reported that negotiations are underway in eastern Syria between the US -led forces and the IS group for the latter's withdrawal from its last redoubt.
Up to 600 fighters could still remain inside, majority foreigners, according to Bali.
"Remnants - that's all they have, remnants - but remnants can be very unsafe", Trump said.
In December, US President Donald Trump said IS militants were "mostly gone" and announced the US would withdraw all of its 2,000 troops from Syria.
The humanitarian pause by the SDF follows reports that IS are holding high-profile western hostages, such as British journalist John Cantlie and popular long-term Syria resident Father Paolo Dall'Oglio, as bargaining chips.More news: Will Saudi Arabia be held to account for Khashoggi's murder?
The SDF official also said high-level Islamic State commanders had not been captured yet.
The withdrawal would also include troops deployed at the U.S. military base at Tanf, near the Syrian border with Iraq and Jordan, they said.
The Observatory says that suspected IS sleeper cells have allegedly assassinated at least 50 civilians and 135 SDF fighters in Kurdish-held territory since August.
The Syrian government has said that the areas controlled by the Kurds would eventually revert to government control.
But in December Washington announced it would pull out all its troops from Syria, sending Kurdish authorities scrambling to mend fences with the regime in Damascus.More news: Taliban not seeking to seize whole of Afghanistan