The US military commander on Thursday offered a grim outlook on the status of ISIS as US President Donald Trump prepared to declare territorial victory over the extremists in Syria and Iraq.
Commander Gen Joseph Votel said the fight was far from over.
“What we are seeing now is not the surrender of ISIS as an organisation but a calculated decision," he said. "They are waiting for the right time for a resurgence.”
Gen Votel, who is leaving his position as head of Central Command in coming weeks, told a Senate committee that retaking territory from the militants was "a monumental military accomplishment but the fight against ISIS and violent extremism is far from over".
He said that the ISIS fighters and their families leaving the group's last pocket of territory were "unrepentant, unbroken and radicalised".
Gen Votel said their territorial surrender was a tactical decision “to preserve the safety of their families and preservation of their capabilities".
He confirmed that the US would not fully withdraw from Syria.
“The US will retain a residual force on the ground in Syria to support its coalition allies and partners as it draws down troop presence,” Gen Votel said.
The White House will probably leave 400 of its 2,000 troops in Syria.
It is also in talks with the Europeans and Nato partners to bring together an observation force for the border between Syria and Turkey.
Gen Votel voiced concern over the thousands of fighters and family members leaving ISIS areas, who are being held in Kurdish-controlled territory inside Syria.
“In my view this is a serious generational problem that, if not handled properly, will sow the seeds of future violent extremism,” he said, urging the international community to do more.
Gen Votel also spoke about the threat of Iran, which he said was trying to become supreme leader of the region "and to use its malign influence”.
On Afghanistan, Gen Votel said there were no orders to withdraw even partially, despite comments from Mr Trump.
He said that any decision to reduce the force there should be done in full consultation with the coalition and the Afghan government.
Lt Gen Kenneth McKenzie will replace Gen Votel after a confirmation by the Senate. The departing general held the job for three years.