Iraqi and Kurdish forces, backed by US-led coalition trainers and air power, have for months been edging toward Mosul, Iraq's second city and home to two million people. The Islamic State group has controlled it since June 2014.
"We see them weakening inside Mosul," said Colonel Chris Garver, a coalition spokesman. "We do see some indications that morale is lower."
For instance, senior commanders are executing subordinates for "failure on the battlefield," Garver said, adding that IS leaders are "not happy with where they are in Mosul."
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Jihadists, concerned that city residents are communicating with Iraqi security forces, have started cutting off internet access.
The same thing happened in Fallujah and Ramadi before each of those cities was recaptured, Garver said.
Still, he cautioned, the eventual fight for Mosul -- which is expected to begin in the coming months -- will not be easy.
"We still anticipate that somewhere between 5,000 or so fighters are inside Mosul," Garver said. "We're still anticipating a tough fight."