The Syrian Observatory for Human Rights, a UK-based war monitor, said the alliance of Kurdish and Arab fighters had pressed forward in the face of tough obstacles.
A U.S. departure makes them more vulnerable to a long threatened attack by neighbouring Turkey, who considers Kurdish fighters to be "terrorists", and dashes their dreams of autonomy.
After capturing Raqqa, the SDF advanced southwards into Deir al-Zor province, attacking the jihadists in territory on the eastern bank of the Euphrates River.
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.
Trump's decision to withdraw about 2,000 U.S. troops has also left Syria's Kurds scrambling for reassurances.
"But their presence in the information space is something which we are going after as vigorously as we go after their presence on the ground".
While Turkey seeks to preserve the territorial integrity of Syria, a resolution of the conflict remains elusive due to the U.S.' support of the YPG, which pursues a separatist agenda in the country.
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"IS launched a counterattack on our forces and we are now responding with rockets, air strikes and direct clashes", SDF spokesman Mustafa Bali told AFP.
The U.S. -led coalition has said it is not discussing the timeline of the U.S. withdrawal from Syria.
More than 79 states and worldwide organisations joined the coalition against ISIS but Monday's fighting was being carried out by Kurdish-led SDF fighters with support from coalition warplanes and artillery.
With U.S. -backed fighters poised to seize the final Syrian sanctuaries held by Islamic State in the coming days, the U.S. military is turning its attention toward a withdrawal of forces in the coming weeks, these people said on Thursday. The US military has warned the group could stage a comeback if the military and counter-terrorism pressure on it is eased.
"The coalition's hard-won battlefield gains can only be secured by maintaining a vigilant offensive against the now-largely dispersed and disaggregated ISIS that retains leaders, fighters, facilitators, resources and the profane ideology that fuels their efforts", Votel added. But holed up in tunnels, trenches, and houses around Baghouz are an estimated 600 diehard ISIS fighters.
IS has suffered substantial losses, but the United Nations says it still reportedly controls between 14,000 and 18,000 militants in Iraq and Syria, including up to 3,000 foreigners.
"That is, if a buffer zone that will safeguard terrorists is envisaged or desired, we are against that", said Cavusoglu, referring to USA demands to ensure protection of Kurdish fighters from the People's Protection Units (YPG) who are allied with Washington in the war against ISIS.