The Obama administration is boxed in by its promise to limit U.S. military engagement against Islamic State extremists, making it tough to agree to Turkey's condition for joining the fight in neighboring Syria.

Turkey and other U.S. allies want the U.S. to create a no-fly zone inside Syrian territory. Doing so would mean embracing one of two options President Barack Obama has long resisted: cooperating with Syrian President Bashar Assad's government or taking out its air defenses, action tantamount to war.

Airstrikes alone might not prevent Islamic militants from carrying out a massacre at a Kurdish border town, but for now the U.S. isn't steering a new course in its expanded, one-month counterterrorism effort in Iraq and Syria.

Demands are rising for the creation of a secure buffer on the Syrian side of its frontier with Turkey as the U.S. and its coalition members plead with the Turks to prevent the fall of Kobani, where the United Nations is warning of mass casualties. A "safe zone" would require Americans and their partners to protect ground territory and patrol the skies, meaning it would have to enforce a no-fly area.

Turkey, an American NATO ally, is demanding such a step for a variety of reasons. A buffer might stem the flow of refugees into Turkish territory. It also could provide Syrian opposition fighters with a staging ground for their rebellion to oust Assad — something that Turkey wants to see happen. The U.S., wary of the implications, wants the focus to remain on defeating Islamic State militants who've captured large areas of northern Syria and Iraq. Yet some of America's closest partners and Obama's fiercest foreign policy critics at home are sympathetic to Turkey's request.

France issued a statement this week announcing its support. The Republican head of the House Foreign Affairs Committee believes Arab countries would shoulder the load. Even Secretary of State John Kerry is describing a no-fly zone as worth examining.

Defense Secretary Chuck Hagel has shown little enthusiasm for the idea. American leaders are open to discussing a safe zone, he said this week, but creating one isn't "actively being considered."

"When it comes to the so-called buffer zone, no-fly zone, they've proposed these for some time," State Department spokeswoman Marie Harf said Friday. "We are not considering the implementation of this option at this time."

For the U.S. military, creating a protected corridor in Syria safe from the Islamic State group's ground attacks and Syria's air force raises obvious red flags. Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, has estimated the endeavor would require hundreds of U.S. aircraft and cost as much as $1 billion a month to maintain, with no assurance of a change in battlefield momentum toward ending the Syrian civil war. That means U.S. enforcement could become open-ended.

The Pentagon learned that lesson in Iraq when it established a no-fly zone over northern Iraq to protect Iraqi Kurds and another over southern Iraq to protect Shiites — both in 1991 in the aftermath of the Gulf War. Those protective zones were enforced by U.S. Air Force and Navy pilots for a dozen years, until the March 2003 invasion of Iraq.

Implementing such zones in Syria would create a direct confrontation with one of the Middle East's most formidable air defenses, a system bolstered in recent years by top-of-the-line Russian hardware. The Syrians possess multiple surface-to-air missiles providing overlapping coverage and thousands of anti-aircraft guns capable of engaging attacking aircraft at lower levels. Moscow infuriated Washington last year when it confirmed that it would sell to Syria its S-300 anti-aircraft missiles, considered to be the cutting edge in aircraft interception technology.

For the Obama administration, the political challenges of a no-fly zone may be even greater.

Given the threat to U.S. pilots, the military would need rescue personnel stationed nearby, perhaps in Turkey or Iraq. If a U.S. flier were shot down, a rescue team would have to put boots on the ground in Syria, which Obama has repeatedly ruled out.

Engaging in direct military action against Assad's government also would severely stretch the United States' already tenuous claims that its intervention in Syria is legal under U.S. and international law. Many members of Congress are challenging the administration's justification for war on the basis of the Bush administration's 2001 authorization to fight al-Qaida. The Islamic State group grew out of the al-Qaida movement, but the two are now enemies. The U.S. has no U.N. mandate to wage war in Syria, against rebels or the government.

To avoid these pitfalls, the U.S. could try to reach an accommodation with Syria's government. But Obama has ruled out this step, too, even if Syrian forces are similarly battling the Islamic State militants, because of their atrocious list of alleged human rights violations and war crimes. These include massacres of civilians and opposition forces and several chemical weapons attacks, according to Western governments and human rights groups.

Washington is searching for an alternative approach with Turkey. On Friday, retired Marine Gen. John Allen, the U.S. special envoy, and his State Department deputy Brett McGurk met in Ankara with senior Turkish officials and NATO's secretary-general and discussed "where we think Turkey can contribute more, especially along the military line of effort," Harf said. She praised Turkey for agreeing to help train and equip moderate Syrian opposition forces and said U.S. military planners would hold further talks with their Turkish counterparts next week.

Kerry, meanwhile, is headed to the Middle East for some "coalition massaging" this weekend, according to U.S. officials. On the sidelines of a Gaza reconstruction conference in Cairo, he will discuss the effort against the Islamic State group with foreign ministers from Egypt, Qatar, Saudi Arabia and other Arab countries pressing for more robust American action. Kerry, however, isn't preparing to present a new direction in the strategy, said the officials, who weren't authorized to speak by name on the matter and demanded anonymity.

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AP National Security Writers Robert Burns and Lara Jakes contributed to this report.