WASHINGTON, July 10 (Xinhua) -- U.S. Secretary of State Rex Tillerson on Monday congratulated Iraq on the liberation of Mosul, hailing the victory as a "critical milestone" in the global fight against the Islamic State (IS) group.

The liberation of Mosul "underscores the success of the international effort led by the Iraqi Security Forces," Tillerson said in a statement.

"We honor the sacrifices of the Iraqi Security Forces and Iraqi people, as well as the Kurdish Peshmerga, in achieving this victory, and express condolences for the many lives lost in the operation," the top U.S. diplomat added.

Iraqi Prime Minister Haider al-Abadi on Monday formally declared Mosul liberated from the IS after nine months of fierce fighting to dislodge the extremist militants from their last major stronghold in Iraq.

"I declare to the whole world the end, failure and collapse of Daesh state, the state of (IS group) terrorism, which they announced it here in Mosul three years ago," Abadi said in a speech in Mosul.

On Monday, Tillerson vowed that the U.S. and its Coalition partners will continue to work closely with the UN to stabilize liberated areas throughout Mosul, supporting the return of displaced civilians to their homes.

He also cautioned that there is still much work to be done to defeat the IS, adding that the Coalition forces will continue to stand with its Iraqi partners to ensure that IS is defeated wherever it remains in Iraq.

Mosul, 400 km north of Iraq's capital Baghdad, came under IS control since June 2014, when government forces abandoned their weapons and fled, enabling IS militants to take control of parts of Iraq's northern and western regions.