The Pentagon says the Islamic State is on the run in east Mosul, and that U.S. airstrikes blasted 10 boats on the Tigris River as Islamic State fighters tried to flee to the western part of the Iraqi city.

"It is a great to not be a member of the ISIL Navy," Navy Capt. Jeff Davis, a Pentagon spokesman, said on Monday.

The Pentagon says U.S. airstrikes have taken out 143 of the boats that the Islamic State uses to move from one side of the city to the other now that all five bridges that cross the Tigris have been destroyed.

"Our message to ISIL is this: 'We will not allow you to get away to the other side of the river, and set up shop there,' " Davis said.

The watercraft make an easy target for U.S. warplanes, and the boats' destruction prevents the Islamic State fighters from escaping and joining the last remnants of the Islamic State who are holed up in the downtown area of the northern Iraqi city.

"When a fighter is on the run, a legitimate combatant is on the run, and is presumably moving to a position where they can continue to fight, they are still a legitimate threat for us," Davis said.

Iraqi forces have essentially defeated the Islamic State on the east side of the river, and the terrorist group is now confined to the western area, which is more densely-populated and presents a bigger challenge to liberating forces.

"We will leave it to them [the Iraq forces] to officially declare when it's liberated, but I will tell you in east Mosul we are well past the tipping point," Davis said, but he cautioned the fight in western Mosul is going to be tough urban battle.