Jim Michaels

USA TODAY

Iraqi security forces are speeding up plans to retake territory seized by the Islamic State, and could launch a major offensive against the militants this winter, according to a senior U.S. defense official.

Iraq's military had been so disorganized that initial preparations called for a counterattack no earlier than next spring. The new plans reflect recent battlefield successes that have forced militant retreats, boosting the confidence of Iraq's government and armed forces, according to the U.S. Central Command official, who is involved in the planning. He asked that he not to be named because he is not authorized to discuss the campaign plans publicly.

Iraq's forces have been aided by airstrikes on the militants from a U.S.-led coalition.

Iranian jets also may be bombing Islamic State fighters just inside Iraq's border with Iran, the Pentagon said, adding that there is no coordination with U.S. forces. Iran has denied conducting such airstrikes.

A centerpiece of the ground offensive against the Islamic State will be a push to retake Mosul, Iraq's second largest city, which is expected to be a complex battle that could take months and involve bloody street fighting.

"Mosul will be and must be by design a decisive campaign objective," Pentagon press secretary John Kirby told reporters recently. "We will be completely ready to support them from the air when it comes to that."

Kirby declined to speculate on when the campaign would commence.

The Central Command official said Iraq's security forces have already begun operations to set the stage for major offensives in Mosul and elsewhere. He cited progress against Islamic State militants in Beiji, a strategic oil refinery in northern Iraq, and parts of Anbar, a province west of Baghdad.

Success in Mosul will hinge largely on the ability of Iraq's central government, composed many of Shiites, to win the cooperation of local police, many of whom are Sunnis, to secure the city. Iraq's military also will have to work alongside Kurdish forces and local Sunni tribes in retaking the city.

An agreement reached this week by Iraq and the Kurds on dividing oil revenue should help forge cooperation between their military forces.

Iraq's new government has been working to win support from Sunnis in the Mosul area, said Sterling Jensen, an assistant professor at the United Arab Emirates' National Defense College in Abu Dhabi.

Iraq's security forces suffered a humiliating defeat in Mosul when Islamic State forces swept through the city last June. The militants captured tanks, artillery and other equipment abandoned by fleeing Iraqi forces.

The new Iraqi government headed by Prime Minister Haider al-Abadi has made efforts to win support from Sunnis to combat the Islamic State, also composed of Sunnis, Jensen said.

"So far, the Iraqi government has been promising Sunnis more support in their fight against (the Islamic State) and has retired a lot of corrupt and ineffective security officials," Jensen said.

The U.S. military has said it will take about a year to rebuild 12 new Iraqi brigades, consisting of about 60,000 soldiers, to replace the forces that collapsed when Mosul fell. Those forces can be used to relieve forces now combating the militants.

The Pentagon is sending 3,000 U.S. troops to Iraq to secure U.S. facilities in the country, protect Americans and advise Iraq's military on ways to rebuild its forces and combat the Islamic State. None of the Americans will be involved in direct combat.