U.S. forces are amassing and training 20,000 to 25,000 Iraqi and Kurdish forces in preparation for a spring attack to retake Mosul, Iraq from an estimated 1,000 to 2,000 fighters of the Islamic State, or ISIS.

In an extraordinary briefing for reporters, an official from U.S. Central Command, or CENTCOM, which oversees the military coalition fight against ISIS in Iraq, detailed the size and composition of a force that the U.S. hopes will be ready for the offensive within as early as five weeks.

The coalition attack force would consist of five Iraqi brigades that the official said must first go through a U.S. training course, in addition to three Peshmerga brigades that would attack Mosul “from the north,” a newly formed “Mosul fighting force” of mostly Mosul police officers, and special operations forces of Iraq’s Counter Terrorism Service – all of whom are under the advise and assist leadership of U.S. forces in Iraq.

Gen. Lloyd Austin, CENTCOM commander, and Joint Chiefs Chairman Gen. Martin Dempsey are still eyeballing April or May for the offensive, the official said, but that timetable could slide if enough Iraqi brigades have not completed the training, or for other factors.

The U.S. official would not say how much territory ISIS fighters now control because, he said, there are varied estimates even among the intelligence community. "I would be hard pressed," said the official. But since operations against ISIS began last summer, Iraqi Security Forces have retaken between 700 and 800 square kilometers.

"Militarily, ISIL is in decline,” said the official, who claimed the coalition had taken three-quarters of one military division’s worth of equipment and resources “off the battlefield.” But the official offered no clearer picture of the estimated sizes and state of ISIS fighters today.

On the north flank of the greater ISIS war, the U.S. military official notably also cited the pressure on ISIS coming from President Bashar al-Assad’s forces as a factor in the group’s decline.

In the north, Syrian rebels are closer to receiving U.S.-led training. A training site in Turkey for Syrian rebels is "ready to go," the official said, but is awaiting a final agreement with the U.S. to be signed. A training site in Saudi Arabia is 60 to 90 days away from being ready to open. Training at a third site in Qatar is still nine months away.