The US government has announced it will deploy 200 more troops to Syria to help rebels fight Isis.
Speaking in the Bahraini capital of Manama, Defence Secretary Ash Carter said the US would send the troops to help the coalition of Kurdish and Arab rebels, who are already supported by 300 American special forces.
The elite personnel have been deployed in the country since 2014 and have recently begun an offensive to push the extreme Islamist movement out of its stronghold in Raqqa.
An offensive to retake Mosul in Iraq is being carried out simultaneously by the Iraqi army.
The two cities are the last major urban centres under Isis control after the group was pushed out of its other major positions – including the symbolically important town of Dabiq in October – over the past year.
Mr Carter said troop reinforcements would include bomb disposal experts and trainers as well as extra special forces, the AFP news agency reported.
He told reporters the coalition against Isis had reached a “critical milestone” as the battle for Mosul edges closer to the Tigris river which divides the city.
“This is a complex mission that will take time to accomplish but I'm confident that Isis days in Mosul are numbered,” he said.
Mr Carter, who is on a tour of the Middle East ahead of Barack Obama’s departure from the White House in January, said the US and the West had to remain vigilant in the fight against extremism in the Middle East saying: “We must be ready for anything”.
It follows the death of a US officer in Syria last month.
Senior Chief Petty Officer Scott Dayton was killed by an improvised explosive device (IED) near the former Isis stronghold of Ayn Issa, 56km from Raqqa on 24 November.
The 42-year-old from Woodbridge, Virginia, had been working with a bomb disposal unit.
Commenting on his death at the time, Mr Carter said it was “a painful remainder of the dangers our men and women in uniform face around the world to keep us safe”.
