WASHINGTON — President Trump said on Wednesday that the last territory in Syria held by the Islamic State would be “gone by tonight,” despite reports of continuing fighting between the extremist group and American-backed local forces in Syria.

It was the second time over the last month that Mr. Trump was ready to declare the liberation of the extremists’ self-declared caliphate — an area once roughly the size of Britain — across Iraq and Syria. In February, at a military base in Alaska, he said the Islamic State’s territory in Syria had been “100 percent” reclaimed; the group no longer controls land in neighboring Iraq.

In Syria, officials with the American-backed Syrian Democratic Forces near the last pocket of extremist-controlled territory said on Wednesday that a group of Islamic State fighters still controlled a sliver of land along the eastern bank of the Euphrates River, where they were holed up with women and children.

As long as that area was still under the Islamic State’s control, “it would be weird to expect an announcement in the next day,” said a Syrian Democratic Forces official, who was not authorized to respond to Mr. Trump’s comments and spoke on the condition of anonymity.