Iraqi troops will begin a ground offensive "in the weeks ahead" to take back swaths of the country seized by the Islamic State (ISIS) group, the U.S. coordinator for the international coalition against the jihadists said Sunday.

"There will be a major counter offensive on the ground in Iraq," top U.S. envoy John Allen said in an interview with Jordan's official Petra news agency.

"In the weeks ahead, when the Iraqi forces begin the ground campaign to take back Iraq, the coalition will provide major firepower associated with that," he added, stressing that the Iraqis would lead the offensive.

Allen dismissed accusations that there has been a delay in the supply of U.S. weapons and training to Iraqi troops on the frontline of the conflict, telling Petra: "The United States is doing all it can to deliver its support as quickly as possible."

The U.S. has been leading an aerial campaign against the jihadists, who have seized swathes of Iraq and Syria and have imposed a brutal form of Islam in the territory.

Jordan, part of the coalition, announced Sunday that it had conducted dozens of air strikes on IS targets after the group burned one of its air force pilots to death and released a gruesome video of the execution.

U.S. Secretary of State John Kerry said the assault was beginning to win back territory and deprive the jihadists of key funds.

There have been 2,000 air strikes on IS since the coalition's formation in August, Kerry told the Munich Security Conference on Sunday.

The air war had helped to retake some 700 square kilometers (270 square miles) of territory, or "one-fifth of the area they had in their control", he said.

The top U.S. diplomat did not specify whether the regained territory was in Iraq or Syria.

But he added the coalition had "deprived the militants of the use of 200 oil and gas facilities... disrupted their command structure... squeezed its finance and dispersed its personnel."