The U.S. plan to step up attacks against militants in Iraq and Syria is being delayed until more Arab governments agree to contribute to the operation, the Pentagon's top officer said Sunday.

President Obama wants to see more specific pledges from allies to join and help pay for the military operation against Islamic State militants before he will give final approval to the Pentagon's war plan, Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, told reporters traveling with him in Europe.

"The president has not yet approved the campaign plan in part" because we have not yet completed our work of building the coalition, especially of Arab nations, Dempsey said. "I think that would be the point at which the campaign plan would go into its next phase."

The delay provides the latest example of the stop-and-go nature of the U.S. effort against Islamic State, which has the potential to cause friction between the White House and the nation's military leadership. Pentagon officials worry about ambivalence on Obama's part regarding involvement in another war in Iraq, despite what the president has said is the risk that Islamic State could one day threaten the United States....