Jim Michaels

USA TODAY

U.S.-led coalition warplanes successfully targeted a flotilla of 90 Islamic State boats being used by the militants to cross the Tigris River in a desperate effort to escape fighting in eastern Mosul, the U.S. military announced Saturday.

The airstrikes occurred as coalition-backed security forces seized the eastern portion of Mosul, Iraq’s second largest city, and began preparations for clearing the western side of the city. The city is divided by the Tigris River.

The strikes targeted 90 boats and three barges between Wednesday and Friday, the coalition said in a statement. Most were being used by the Islamic State, also called ISIS or ISIL, to escape from the eastern part of the city, which has been secured by Iraqi forces in recent days. Since the Mosul operation began in October, the coalition has hit 112 watercraft on the Tigris River in Mosul.

"We believe this was a desperate attempt to retrograde ISIL fighters now that the Iraqi security forces own the eastern bank of every bridge in Mosul," Col. John Dorrian, a coalition spokesman, said in the statement.

U.S.-coalition aircraft earlier disabled five bridges that cross the Tigris River in an effort to prevent militants from sending fighters and car bombs across the river to attack Iraqi forces as they fought through the eastern part of the city.

U.S.-backed Iraqi security forces are now preparing to retake the western part of the city, a densely populated area with thousands of civilians. Iraqi forces are holding the eastern part of the city as additional forces press in from the north and south.

The Mosul offensive was launched three months ago in an effort to recapture the Islamic State’s remaining stronghold in Iraq. A U.S.-backed force of about 45,000 Arabs and Kurds is simultaneously closing in on Raqqa, the Islamic State’s defacto capital in neighboring Syria.

It is not clear how many militants were killed in the strikes, but the coalition wants to prevent fighters from escaping Mosul and fleeing to Raqqa.