Senior U.S. military commanders have become so pleased with events in Iraq that they expect troop levels (search) to drop to about 100,000 by early next year, depending on the persistence of the insurgency and local security-force training, defense sources confirmed to FOX News.

"We're hitting our goals," one senior U.S. commander told FOX News, adding that the military did not want to be overly optimistic about developments in Iraq.

Sen. Norm Coleman (search), who recently visited Iraq and talked with Gen. David Petraeus (search), the top American military trainer in Iraq, said Iraqi security forces were taking more of a lead role in protecting their country, with American forces shifting to the background.

Coleman acknowledged that there was still work to be done.

"It's not an exit strategy, it's a success strategy," said the Minnesota Republican. "Success is this vision that I talked about — Americans embedded with [an] Iraqi army that is on the front line dealing with a policing operation and dealing with military operations and wiping out insurgency."

The possible troop-level reduction was first reported Monday in The New York Times. Click here to read that story.

Military commanders in Iraq said joint exercises between U.S. troops and Iraqi forces were disrupting the insurgency. Since January's elections, violence against U.S. troops has been reduced by 20 percent, but insurgents have mounted more attacks on Iraqi forces and civilians.

Several top associates of Iraq's most-wanted man, Abu Musab al-Zarqawi (search), have been captured or killed in recent weeks. Zarqawi is the Jordanian militant whose Al-Qaeda-affiliated network has claimed responsibility for many of the deadliest attacks.

American commanders say the insurgency's latest tactics, as shown by the many-pronged conventional assault on the Abu Ghraib (search) prison complex the night of April 2, which wounded 44 Americans and 13 Iraqi prisoners, means it will take longer for insurgents to plan and carry out attacks.

Senior commanders say the insurgency is still mostly driven by Iraqis, but add that small numbers of foreign fighters, who carry out most suicide bombings, are sneaking into the country, mainly from Syria.

U.S. officials said that as more Iraqi troops have taken the front lines against the insurgency, more Iraqi civilians and politicians have cooperated with multinational forces.

But no official would give a precise timetable about when American forces would withdraw or how many troops would be involved.

FOX News' Bret Baier contributed to this report.