U.S.-backed Syrian militias on Tuesday relaunched their campaign to seize the last territory the IS group controls in the east near the border with Iraq.
The Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab militias, had paused their battle after Turkey launched an assault in January against their northern Afrin region.
"We have rearranged our ranks," said Lilwa al-Abdallah, spokeswoman for the offensive in oil-rich Deir al-Zor province.
IS militants stepped up attacks in the region in recent weeks in a bid to reorganise, she told a news conference at an oilfield on the eastern bank of the Euphrates river.
"Our heroic forces will liberate these areas and secure the border...We welcome the support of the Iraqi forces."
Another SDF official in Deir al-Zor said that coordination with Iraqi troops had increased and that neither side would cross the border.
U.S. Defense Secretary Jim Mattis said last week that he expected a "re-energised" effort soon against the ultra-hardline militants in eastern Syria.
Syrian fighters, backed by U.S. air strikes and troops, have dealt heavy blows to IS but the latter still hold a swathe of land along the desert frontier with Iraq. They are widely expected to revert to guerrilla tactics if they lose the last remnants of their once self-styled "caliphate".
The SDF alliance, spearheaded by the Kurdish YPG, has seized vast tracts of territory from IS in the north and east with Washington's help.
Turkey's offensive against the YPG in Afrin, which Ankara sees as an extension of an outlawed Kurdish insurgency at home, led to a pause in the campaign against IS, the Pentagon said in March.