As U.S. airstrikes in Iraq extend into their second week, President Obama finds himself with few good options.

After withdrawing U.S. forces from Iraq in 2011, Obama now must decide whether to stretch the limited rationales he's given for airstrikes or pull back and risk allowing fighters with the Islamic State to secure more territory.

The Islamic State's release of a grisly video showing the beheading of an American photojournalist Tuesday only ratcheted up the pressure on the president.

In launching the strikes in early August, Obama said the bombing would be targeted and limited to two main missions: providing humanitarian assistance and protecting U.S. personnel and facilities.

Obama’s first military action fit neatly in those two categories when he directed the U.S. military to intervene to help thousands of Yazidis to escape a mountaintop where they were trapped by Islamic State fighters and struck convoys of militants drawing uncomfortably close to the U.S. consulate and joint command center in Irbil.

On Monday, Obama pushed his authorization a bit further to justify U.S. airstrikes that helped Iraqi and Kurdish forces dislodge Islamic extremists from the Mosul dam.

Pentagon officials said that airstrike operation fell under the mission of protecting U.S. personnel in Baghdad more than 200 miles away because any damage to the dam could flood the surrounding area, threatening U.S. facilities and creating a humanitarian disaster.

But with the worst, immediate threats from the Islamic State now out of the way, the rationale for further airstrikes gets far trickier from here on out, leaving Obama vulnerable to charges of mission creep.

Pentagon spokesman John Kirby defended U.S. action from charges of mission creep and said any additional airstrikes would remain limited — at least for now.

"We believed that, should the dam remain in control of [the Islamic State] — whose intentions are obviously not perfectly clear and certainly not in the best interests of the people of Iraq — if that dam was to blow or they were to open and flood the gates, that it could have an effect as far south as Baghdad,” he said.

Kirby declined to discuss whether the U.S. military would launch airstrikes in Tikrit, or anywhere else where it would be difficult to justify them under the two missions Obama has set out so far.

“I’m not going to speculate about future operations,” he told reporters Tuesday.

The U.S. would continue to provide strategic advice to the Iraqi army whenever needed, he said, stressing that the U.S. is not the Iraqi air force and the Iraqi military would have to step up in order to hold the country together.

“We can assist where we can, but this is theirs to fight,” he said.

Boxing him in even further, most Democrats on Capitol Hill don’t want to have to vote for broader authorization for military action in Iraq.

Members of Congress across the political spectrum hit Obama for failing to seek congressional approval for U.S.-led military action in Libya, but right now lawmakers facing tough re-elections a little more than two months away want to avoid voting on the unpopular prospect of sustained military action in Iraq.

At the same time, some congressional Democrats and several members of Obama’s former national security team have called for a broader U.S. commitment to stabilize Iraq, in addition to a more cohesive counter-terrorism mission.

Over the weekend, Rep. Eliot Engel, D-N.Y., said the U.S. might need ground troops to fight the Islamic State and said it would be a mistake for the U.S. to turn its back on the terrorists groups and “put our head in the sand.”

Fellow Congressional Rep. Tulsi Gabbard, D-Hawaii, one of the first female combat veterans in Congress, said on Sunday that the administration’s counterterrorism mission “has been lost” and took issue with the administration’s argument that the U.S. airstrikes on the Islamic State were a response to a humanitarian crisis.

“We’re missing a critical question here,” she told ABC News. Gabbard, who also serves as a vice chairwoman for the Democratic National Committee, said she joined the Army National Guard after the 9/11 terror attacks because U.S. leaders vowed to “take out Islamic extremists wherever they are.”

“That mission has been lost. [A White House official] said the air strikes are not a campaign against [the Islamic State]. If our mission is not about taking out extremists, then I think we’ve got a real problem here,” she said.

That scorching came after a string of former Obama administration officials and high-ranking diplomats accused the president of failing to implement a coherent Middle East strategy.

Those critics range from former Secretary of State Hillary Clinton to Obama’s former national security adviser Gen. Jim Jones to his former Iraqi Ambassador Chris Hill and Ali Khedery, the longest continuously serving U.S. official in Iraq.