Turkey and the United States have been discussing plans to try to rout Islamic State militants from Syrian territory along the Turkish border, creating a safe haven for thousands of Syrian refugees and cutting off the militant group's main route for getting recruits and supplies.
The move builds on unprecedented actions by Turkey during the last week to battle IS militants in neighboring Syria and Iraq, while also cracking down on Kurds across the country.
Obama administration officials said on July 27 that discussions with Turkey about an IS-free zone have focused on a 110-kilometer stretch of land under IS control.
The United States has been conducting airstrikes against IS miitants there.
U.S. officials say those air strikes are expected to accelerate now that Turkey is allowing the U.S. to launch air strikes from Turkish soil against IS militants.
But the U.S. officials said an agreement between Turkey and the United States on the so-called "secure zone" has not yet been finalized.
Turkey's President Recep Tayyip Erdogan said on July 28 that a "secure zone" in northern Syria would pave the way for the return of 1.7 million Syrian refugees currently being sheltered in Turkey.
Meanwhile, other Turkish officials have said the goal is not to create a "safe zone" for people who are fleeing Syria's civil war, but rather, "to clear the border and keep the border closed" to IS militants.
Erdogan on July 28 also urged the NATO alliance to be prepared to help Turkey in its fight against IS militants in Syria and Kurdish rebels based in northern Iraq.
His remarks came as NATO Secretary-General Jens Stoltenberg opened a rare emergency meeting about the IS threat at NATO headquarters in Brussels at the request of Turkey.
Stoltenberg on July 28 expressed condolences to the Turkish people and government over recent deadly attacks. The NATO chief said "terrorism in all its forms" can never be justified.
In Washington, State Department spokesman John Kirby said that any joint military efforts with Turkey would not include the imposition of a no-fly zone over northern Syria -- something Turkey has long sought.
While details of the buffer-zone plan have not yet been worked out, Turkish Prime Minister Ahmet Davutoglu said Ankara and Washington have no intention of sending ground troops into Syria but want to see Syria's moderate opposition forces replace IS near the Turkish border in the buffer zone.
"We do not want to see Daesh on Turkey's borders," he told Turkish TV, using an Arabic-language term for Islamic State. "Moderate forces like the Free Syrian Army will be strengthened, a structure will be created so that they can take control of areas freed from [IS], air cover will be provided. It would be impossible for them to take control of the area without it."
The discussions come amid a major shift in Turkey's approach to IS. After months of reluctance, Turkish warplanes started striking militant targets in Syria last week, and allowed the United States to launch its own strikes from Turkey's strategically located Incirlik Air Base.
Turkey has also called a meeting of its NATO allies for July 28 to discuss threats to its security and its air strikes. Davutoglu said "NATO has a duty to protect" Turkey's border with Syria and Iraq, and that Ankara will seek the alliance's support for its actions at the meeting in Brussels.
But a Turkish-driven military campaign to push IS out of territory along the Turkish border is likely to complicate matters on the ground.
U.S.-backed Kurdish fighters in Syria, who have been the most successful in the war against IS, control most of the 910-kilometer boundary with Turkey, and have warned Ankara against any military intervention in northern Syria.
IS itself controls roughly a 95-kilometer stretch of that border, wedged between Turkish-backed insurgents with Islamist ideologies and Kurdish forces from the People's Protection Unit, known as the YPG.
The Turkish-U.S. plan raises the question of which Syrian rebel forces would be involved in a ground operation against IS. The United States has long complained about having no reliable partners among them.
Defense Secretary Ash Carter acknowledged earlier this month that the United States has only 60 trainees in a program to prepare and arm thousands of moderate Syrian rebels in the fight against IS militants.
One administration official, without elaborating, said the U.S.-led coalition was looking to anti-IS forces such as Syrian Kurds and the Free Syrian Army to do the fighting on the ground.