Islamic State militants are "living their final moments" in the last enclave they hold near the Iraqi border, where US-backed Syrian Democratic Forces are attacking them, an SDF official says.
A defeat of the jihadists in the enclave would wipe out IS's territorial foothold on the eastern bank of the Euphrates River.
A spokesman for the US-led coalition said on Sunday the SDF were making "great progress ... but the fight continues".
The SDF, a coalition of militias led by the Kurdish YPG, have driven IS from a swathe of northern and eastern Syria with the help of the US-led coalition over the past four years.
Mustafa Bali, SDF media office head, said the SDF had stepped up attacks in the past two days and taken control of the area between the IS enclave and the Iraqi border, cutting off an escape route.
"They are living the final moments and realise that this battle is the battle to eliminate them," he added.
President Donald Trump last month announced he would withdraw US forces from Syria, declaring they had succeeded in their mission to defeat IS and were no longer needed.
Since then, US officials have given mixed messages. On Friday, the US-led coalition said it had started the pullout, but officials later said only equipment, not troops, were going.
Colonel Sean Ryan, the coalition spokesman, said: "The SDF is making great progress and continues to liberate more territory once held by ISIS (Islamic State), but the fight continues.
"The lasting defeat of ISIS is still the mission and they still present a very real threat to the long-term stability in this region, so it is not over yet."
The US decision has injected new uncertainty into the eight-year-old Syrian war and spurred a flurry of contacts over how the security vacuum will be filled in the swathe of northern and eastern Syria where the US forces are now stationed.
While Turkey aims to pursue the Kurdish forces allied with the US, the Russia- and Iran-backed Syrian government sees a chance to recover extensive territory.
The Syrian government hopes dialogue with Syrian Kurdish groups will intensify, a minister said on Sunday, signalling support for talks the Kurds hope will lead to a political deal between two of the main players in the multi-sided war.
The Kurds have sought Russian meditation for the talks with President Bashar al-Assad's government, part of their strategy to fill a vacuum that will be left when US forces withdraw.
IS still holds territory on the western bank of the Euphrates, between areas controlled by the Syrian government and its Russian and Iranian-backed allies.
Australian Associated Press
