No "shock and awe' in Syria

The US military's Central Command is due to brief Obama on its plans on Wednesday. Hagel said those plans envision striking the militant group's safe havens in Syria to knock out infrastructure, logistics and command capabilities.

Dempsey said the strikes would degrade the group's capabilities as broader efforts get under way, including training of more than 5,000 Syrian rebels.

"This won't look like a 'shock and awe' campaign because that's simply not how (the Islamic State militants' group) is organized. But it will be a persistent and sustainable campaign," Dempsey told the Senate Armed Services Committee.

"Shock and awe" was a term popularly used to describe the initial air assault on Baghdad in the US campaign to oust Saddam Hussein in 2003, and refers to use of overwhelming force to undermine an enemy's will to fight.

Congress is expected to approve this week a request from Obama for authorization to arm and train moderate Syrian rebels, one part of his program.

Still, Hagel acknowledged the number of Syrian fighters that could be trained over the course of the year would only put the opposition on a path to roll back Islamic State fighters.

"Five thousand alone is not going to be able to turn the tide. We recognize that," Hagel said.

The Senate hearing was repeatedly interrupted by anti-war protesters, shouting slogans such as, "There is no military solution." One protester was escorted out of the room while holding a sign that read: "More war = More extremism."

Senator Angus King of Maine, expressing concern that the United States would be drawn into interminable fights against extremist groups around the world from Iraq to Syria to Africa, said: "This is geopolitical Wack-a-mole."