The Pentagon is lauding the liberation of Fallujah by Iraqi government forces and says the fall of the Islamic State's last stronghold in Iraq's Anbar province extends the group's losing streak, while underscoring its inability to score any significant success on the battlefield.

"There has been no strategic victory for ISIS in over a year now. We have seen them, in fact, lose significant territory over and over again," said Pentagon spokesman Capt. Jeff Davis, ticking off a string of defeats for the Islamic State since the beginning of 2015.

"Fallujah, Ramadi, Rutbah, Hiit, Sinjar, Baiji on the Iraq side. In Syria we've seen them lose Al Hal, Shaddadi, Tishrin Dam and now the stranglehold on Manbij, and they will soon lose that as well."

Davis said Fallujah has been completely cleared of Islamic State fighters, but the slow work of disarming booby-traps and improvised mines continues.

He said the Islamic States forces were dug in intially. But once Iraqi forces entered the city last month, backed up by more than 100 coalition airstrikes, resistance crumbled.

"They absolutely put up a fight, they absolutely resisted," Davis told reporters at the Pentagon. "But I think to some extent once they go through the hard candy shell and into the chewy center, things went much more quickly."

Defense Secretary Ash Carter issued a statement congratulating Iraqi Prime Minister Haider Al Abadi and the Iraqi people for freeing Fallujah, calling it "another milestone in our joint efforts to accelerate ISIL's defeat."

Carter also acknowledged allegations of severe human rights abuses of some of the people who fled Fallujah during the siege of the city, and called for a thorough investigation by the government of Iraq.

The Pentagon is now focusing on the major objective in the anti-Islamic State campaign: Mosul, the group's de facto capital in Iraq.

Pentagon officials have steadfastly refused to predict when that offensive will happen, and say it depends on decisions that are being made by the Iraqi government.

Davis also cautioned that the latest victory in Fallujah, while significant, was no indication the Islamic State is on the ropes. "I don't want to oversell the fact that it's all done," he said. "There is still significant territory that they hold, over half of what they held at their high mark."