Defense Secretary James Mattis said Sunday the U.S. has switched to “annihilation tactics” against the Islamic State and is focused on completely surrounding the militants instead of moving them from place to place.

“Our strategy right now is to accelerate the campaign against ISIS. It is a threat to all civilized nations. And the bottom line is we are going to move in an accelerated and reinforced manner, throw them on their back foot,” Mattis said in a televised interview on CBS News’ “Face the Nation.”

He said the goal was to take out the militants before they could flee to neighboring countries.

“Our intention is that the foreign fighters do not survive the fight to return home to North Africa, to Europe, to America, to Asia, to Africa. We’re not going to allow them to do so. We’re going to stop them there and take apart the caliphate,” Mattis said.

U.S.-backed forces in Iraq have launched an offensive to reclaim the last ISIS-held districts in Mosul, according to The Wall Street Journal. The assault on the outskirts of the western part of the city started Saturday. The militants are expected to put up one last fight before coalition forces move in.

Iraqi troops will have to do battle on foot because the armored vehicles are too big for Mosul’s narrow streets and back alleys. The part of the city is still filled with around 200,00 civilians.

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