British PM says IS territorial defeat 'historic milestone'

"The liberation of the last Daesh-held territory is a historic milestone that would not have been possible without their commitment, professionalism and courage," she said in a statement, using an Arabic acronym for IS.

"I commend the tireless work and extraordinary courage of the British forces and our global coalition partners that have fought Daesh in Syria and Iraq."

Kurdish-led forces among the 79 international partners pronounced the death of the nearly five-year-old "caliphate" Saturday after flushing out IS jihadists from their last redoubt in Baghouz, eastern Syria.

Britain, which has suffered from multiple terrorist attacks in recent years by perpetrators said to have been inspired by IS, has vowed to remain focused on fighting the group and securing the region.

"We must not lose sight of the threat Daesh poses and the government remains committed to eradicating their poisonous ideology," May added.

"We will continue to do what is necessary to protect the British people and our allies." Britain has committed nearly 1,400 military personnel to the region to support local forces, while the Royal Air Force has conducted more than 1,600 air strikes in Iraq and Syria, according to government figures.

Western allies in the US-led coalition are negotiating with Washington on the residual force that will remain after most of the American soldiers are brought home.

"We cannot be complacent," British Foreign Secretary Jeremy Hunt warned Saturday.

"Even without territory, Daesh and its poisonous ideology will continue to pose a threat to the people of Iraq and Syria, as well as to the wider world.

"The international community must remain firm in its determination to counter and defeat it."