The U.S. military's top officer said Wednesday that almost half of Iraq's army is incapable of working  against the Islamic State militant group, while the other half needs to be rebuilt with the help of U.S. advisers and military equipment. 

Chairman of the Joint Chiefs of Staff Gen. Martin Dempsey made the remarks to reporters while traveling to Paris to meet with his French counterpart to discuss the situation in Iraq and Syria. The general said that U.S. assessors who had spent the summer observing Iraq's security forces concluded that 26 of the army's 50 brigades would be capable of confronting the Islamic State, also known as ISIS. Dempsey described those brigades as well-led, capable, and endowed with a nationalist instinct, as opposed to a sectarian instinct. 

However, Dempsey said that the other 24 brigades were too heavily populated with Shiites to be part of a credible force against the Sunni ISIS. 

Sectarianism has been a major problem for the Iraqi security forces for years and is in part a reflection of resentments that built up during the decades of rule under Saddam Hussein, who repressed the majority Shiite population, and the unleashing of reprisals against Sunnis after U.S. forces toppled him in April 2003. Sunni resistance led to the relatively brief rise of an extremist group called Al Qaeda in Iraq, led by the late Abu Musab al-Zarqawi. That group withered but re-emerged as the Islamic State organization, which capitalized on Sunni disenchantment with the Shiite government in Baghdad.

On Tuesday, Dempsey told the Senate Armed Services Committee that he would consider recommending the return of ground forces to Iraq if an international coalition sought by the Obama administration proves ineffective. 

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On Wednesday, Dempsey said no amount of U.S. military power would solve the problem of ISIS's takeover of large swaths of northern and western Iraq. The solution, he said, must begin with formation of an Iraqi government that is able to convince the country's Kurdish and Sunni populations that they will be equal partners with the Shiites in Iraq's future.

"I'm telling you, if that doesn't happen then it's time for Plan B," he said. He didn't say what that would entail.

Dempsey also said that ISIS fighters in Iraq have reacted to weeks of U.S. airstrikes by making themselves less visible, and he predicted they would "literally litter the road networks" with improvised explosive devices, or IEDs, in the days ahead. That, in turn, will require more counter-IED training and equipment for the Iraq army, he said.

According to the general, a renewed U.S. training effort might revive the issue of gaining legal immunity from Iraqi prosecution for those U.S. troops who are training the Iraqis. The previous Iraqi government refused to grant immunity for U.S. troops who might have remained as trainers after the U.S. military mission ended in December 2011.

"There will likely be a discussion with the new Iraqi government, as there was with the last one, about whether we need to have" Iraqi lawmakers approve new U.S. training, he said. He didn't describe the full extent of such training but said it would be limited and he believed Iraq would endorse it.

"This is about training them in protected locations and then enabling them" with unique U.S. capabilities such as intelligence, aerial surveillance and air power, as well as U.S. advisers, so they can "fight the fight" required to push the Islamic State militants back into Syria, Dempsey said.

A Pentagon plan for training Syrian rebels is another, more controversial element of the plan, which also includes potential airstrikes in Syria; building an international coalition to combat the Islamic State group in Syria and Iraq; and efforts to cut off finances and stem the flow of foreign fighters to the Islamic State group.

President Obama is to be briefed on the planned campaign against ISIS Wednesday in Tampa, Florida, when he meets with Gen. Lloyd Austin, head of U.S. Central Command, which manages U.S. military operations and relations across the Middle East.

The Associated Press contributed to this report.