Flashes of disagreement over how to fight the Islamic State are mounting between President Obama and U.S. military leaders, the latest sign of strain in what often has been an awkward and uneasy relationship.

Even as the administration has received congressional backing for its strategy, with the Senate voting Thursday to approve a plan to arm and train Syrian rebels, a series of military leaders have criticized the president’s approach against the Islamic State militant group.

Retired Marine Gen. James Mattis, who served under Obama until last year, became the latest high-profile skeptic on Thursday, telling the House Intelligence Committee that a blanket prohibition on ground combat was tying the military’s hands. “Half-hearted or tentative efforts, or airstrikes alone, can backfire on us and actually strengthen our foes’ credibility,” he said. “We may not wish to reassure our enemies in advance that they will not see American boots on the ground.”

Mattis’s comments came two days after Army Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, took the rare step of publicly suggesting that a policy already set by the commander in chief could be reconsidered.

Despite Obama’s promise that he would not deploy ground combat forces, Dempsey made clear that he didn’t want to rule out the possibility, if only to deploy small teams in limited circumstances. He also acknowledged that Army Gen. Lloyd Austin, the commander for the Middle East, had already recommended doing so in the case of at least one battle in Iraq but was overruled.

View Graphic Here's how Congress voted on President Obama’s plan to train and equip moderate Syrian rebels.

The White House and Pentagon have scurried this week to insist there is no hint of dissent in the ranks, though in some cases their efforts have focused only more attention on the issue.

Defense Secretary Chuck Hagel tried to reassure the House Armed Services Committee on Thursday that civilian and military leaders at the Pentagon were in “full alignment” and in “complete agreement with every component of the president’s strategy.”

Some lawmakers were skeptical. Rep. Howard P. “Buck” McKeon (R-Calif.), chairman of the Armed Services Committee, suggested that Obama should listen more closely to his commanders. “I think it’s very important that he does follow the advice and counsel that he receives, the professional advice of the military. They are the ones best suited to do that.”

“I realize he’s commander in chief, he has the final say and the final obligation and responsibility,” McKeon added. “I would also request that he not take options off the table.”

Obama’s strategy received a boost with the Senate’s passage of his plan to train and arm about 5,000 Syrian rebels to help fight the Islamic State, a jihadist movement that controls large parts of Iraq and Syria and has threatened to destabilize much of the region.

The 78-22 vote in the Senate came just a day after the House approved its own measure.

Since Aug. 8, the U.S. military has launched 176 airstrikes against Islamic State targets in Iraq. Obama has signaled the military will expand the strikes into Syria, but it is unclear when that new phase will begin.

During a House hearing on Thursday, former gen. James Mattis, previously commander of U.S. Central Command, said Islamic State militants have captured America's focus right now but that it is important to keep a broader view of global terrorism. (AP)

Hagel testified Wednesday that he and Dempsey had approved a plan to conduct strikes against the Islamic State in Syria, and that Obama had received a briefing from Austin that same day at U.S. Central Command headquarters in Tampa.

When asked if the president had endorsed the plan, however, Hagel acknowledged that Obama had not but did not elaborate.

Divisions between Obama and his generals have become a recurring feature of his presidency. In 2009, shortly after Obama took office, Pentagon leaders pressured the new president — who had run on a platform of ending the war in Iraq — to deploy a surge of troops to Afghanistan to rescue the faltering fight against the Taliban.

After a lengthy and tense internal debate, Obama did send more troops, but not as many as some commanders wanted. At the White House, Obama’s top aides privately expressed frustration that the Pentagon had tried to restrict his choices to get the result the military preferred.

At the Pentagon, military commanders expressed their own frustration last year as Obama weighed whether to take action in Syria following the determination that President Bashar al-Assad had employed chemical weapons against civilians. Although the Pentagon had internal disagreements about whether military action was warranted, there were widespread concerns that Obama was on the verge of ordering strikes without articulating goals or a clear strategy.

This time around, The White House and Pentagon agree on the basic outlines of a strategy to attack the Islamic State — one that centers on arming and training proxy forces, including Syrian rebels, Kurdish fighters and the Iraqi army, backed by U.S. and allied air power.

But the Pentagon is eager to retain the option of deploying small numbers of Special Operations forces to the front lines to help the proxy troops or to call in airstrikes from close range.

Mindful of the president’s campaign pledge to end the last war in Iraq, which led to the withdrawal of all U.S. military forces in December 2011, Obama and his aides have insisted since May that he will not send Americans back into combat there.

But as the conflict with the Islamic State has deepened, and 1,600 U.S. troops have deployed to fill advisory and other roles, the White House has struggled to reconcile that reality with its prior statements that Obama would not put “U.S. boots on the ground” in Iraq.

Military leaders have increasingly suggested that Obama’s political promises are restricting their ability to fight. On Wednesday, former defense secretary Robert M. Gates, still an influential figure at the Pentagon, bluntly criticized his former boss.

“There will be boots on the ground if there’s to be any hope of success in the strategy,” Gates said in an interview with CBS News, adding that “the president in effect traps himself” by repeating his mantra that he won’t send U.S. troops into combat.

There are signs that the White House is becoming more flexible. Antony Blinken, the deputy national security adviser, allowed Thursday that “there may be cases where American advisers would go with some of the forces on the ground” or help “to call in some air power” — the kind of leeway the Pentagon wants.

In an interview with MSNBC, Blinken insisted that such deployments would not amount to combat “where Americans are on the ground leading the fight. That is not going to happen. That’s not part of this campaign. The president’s been clear about that.”