A senior U.S. general suggested Wednesday that the operation to retake Mosul in Iraq could last more than one year and feature a bloody combat.

"The operation we did to Manbij, in northern Syria, took about 71 days to complete, from start to finish," Central Command Commander Gen. Joseph Votel said during a panel discussion at the Center for American Progress, a Washington think tank, comparing Manbij to other Daesh strongholds Raqqa and Mosul.

"Raqqa, the self-proclaimed capital of Daesh, is about three times the size of Manbij and Mosul is about three times the size of Raqqa," Votel added suggesting that Mosul is the hardest stronghold of Daesh militant group to capture.

Iraqi and U.S. officials have said the offensive would last for weeks or months.

Votel said Daesh is "an adaptive enemy" that is able to adjust to unfolding conditions that could cause a "fierce combat".

Responding to a question about reports that the offensives to retake Raqqa in Syria and Mosul might happen simultaneously, Votel did not confirm or deny those speculations but underscored the importance of a "simultaneous application of pressure" on both cities under Daesh control.

"I think what we've seen is when we apply pressure on Daesh forces, they do squirt out, they try to go to other locations, they move leaders, they move the bulk of their forces, they try to relocate some of their operations," Votel said.

Speaking just hours before Votel, Gen. Gary Volesky, commander of the U.S-led coalition's land component, told reporters that Daesh leaders and fighters are leaving Mosul.