ISIS have been defeated in the key Iraqi town of Hit, officials said on Thursday.

The elite Counter-Terrorism Service (CTS) of the Iraqi military "completely liberated" the strategically vital town after a month-long assault.

The victory means the US-led coalition can build on other recent gains in Iraq as the town, 150km west of Baghdad, is on a key supply route between ISIS territory in Iraq and Syria.

At least 20,000 civilian fled the central Iraqi town after the assault began in mid-March. Thousands more are expected to be trapped inside the town, which made the protracted campaign more difficult because of fear of civilian casualties.

"Toward the end, there was nothing here, no doctors for my children or my mother. We just stayed in our houses," a resident identifying himself as Abu Muhammed said according to AP.

"We just had tea and sugar left in the kitchen at the end."

ISIS fighters put up heavy resistance after sustained air strikes from coalition warplans. CTS commander General Abdul Ghani al-Asadi said ISIS fighters were overhead as saying "this is our headquarters and we will never leave this area" in intercepted radio communications.

The military leader said the town represented the group's capital in the Anbar province in western Iraq.

Another reason the offensive took so long was because forces had to pulled from Anbar to protect Iraqi prime minister Haider al-Abadi from protests launched by the radical Shia cleric Moqtada al-Sadr. 

The coalition forces will now begin clearing the town of the remnants of ISIS rule.