Patience with the Obama administration's approach to fighting the Islamic State wore thin Tuesday on Capitol Hill as lawmakers pressed Pentagon officials to justify their plans.

"The lack of a coherent strategy has resulted in the spread of [the Islamic State] around the world," Senate Armed Services Chairman John McCain, R-Ariz., said.

"[The Islamic State] is not 10 feet tall. It can be and must be defeated. But that will not happen if we continue to delude ourselves about the capabilities of the current campaign."

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Defense Secretary Ashton Carter and Joint Chiefs of Staff Chairman Gen. Martin Dempsey defended the administration's strategy before McCain's committee a day after President Obama, in a visit to the Pentagon, cited progress in the campaign but cautioned of a long road ahead.

Obama also repeated his assertion that the military was not the only solution to defeating the Islamic State, and that local forces must take the lead on the battlefield.

"This will not be quick. This is a long-term campaign," he said. "[The Islamic State] is opportunistic and it is nimble. In many places in Syria and Iraq, including urban areas, it's dug in among innocent civilian populations. It will take time to root them out — and doing so must be the job of local forces on the ground, with training and air support from our coalition."

Dempsey, in his testimony to the committee, echoed the president: "I do believe that this is a generational challenge," he said.

But the Islamic State has survived, and in some cases gained ground, in spite of a yearlong bombing campaign by the U.S.-led coalition, millions of dollars worth of arms supplies and efforts by thousands of U.S. and allied advisers to train Iraqi troops and diplomacy aimed at isolating the extremist group. And there's a growing concern among military experts that the group will become more durable and difficult to defeat if the tide isn't turned quickly against it.

Obama's comments at the Pentagon "reveal a disturbing degree of self-delusion that characterizes the administration's thinking," McCain said.

Republican lawmakers have been pressing the president for months to at least embed U.S. ground troops with Iraqi forces and to take a stronger hand against Syrian President Bashar Assad. But the administration has resisted on both points, and that impasse is a major reason the GOP-led Congress has not moved on Obama's request to pass a resolution formally authorizing the military campaign.

"I can tell you that I have not recommended that," Dempsey said about embedding U.S. advisers. "The environment is simply not set up to do that."