The U.S. says it sees signs of fear and desperation among the Islamic State forces holding the northern Iraqi city Mosul, as Iraqi forces, with U.S. backing, prepare for an assault that still appears months away.

"We see them weakening inside Mosul," said U.S. military spokesman Army Col. Chris Garver at the Pentagon. "We do see some indications that morale is lower."

"We know that they have started cutting off Internet access and really access to the outside world for the citizens inside Mosul, and we know that they're afraid that Iraqi citizens inside Mosul are going to communicate with the [Iraqi Security Forces]," he said.

Garver said Islamic State forces appear to be preparing for the eventual loss of Mosul, their de facto capital in Iraq. "We've seen that fear in ISIS, in Daesh," said Garver.

Among the indications of desperation are continued executions of lower level Islamic State leaders by commanders displeased with battlefield losses. It is the same pattern the U.S. military observed in other Iraqi cities in the months before they fell to Iraqi Security Forces.

"We saw that in Ramadi, we saw that in Fallujah. We're seeing those reflections, as we call it ... we see those indicators inside Mosul as well," he said.

But Garver also gave several indications the eventual Iraqi offensive is still months off. For one thing, the 560 U.S. troops that have been ordered to prepare the logistics hub that will be the command post for the offensive have yet to be deployed to the captured Iraq air base in Qayyarah.

Garver said U.S. troops will have a mammoth task converting the deserted airfield into a functioning base with housing, tents, roads, and ammunition storage facilities, so it can serve as a staging area for thousands of Iraqi forces. He said those forces will conduct what he says it expected be to be a "tough fight" to dislodge the estimated 5,000 Islamic State fighters from Iraq's second largest city.

"All eyes are kind of focused on Mosul right now," said Garver, who added that the loss of Mosul would deal the Islamic State both a physical and psychological defeat. "Their reputation, as they try to manage it, is going to take a big hit when Mosul does fall."