Turkey and the United States have joined forces in a campaign to drive the Islamic State in Iraq and the Levant back from Turkey's southern border and cinch off the terrorists' vital corridors for weapons and fighters.

The mission to create an ISIL-free zone in northern Syria is still in the planning phase, U.S. officials said Monday, and the new strategy for combating the scourge that has proclaimed a Muslim "caliphate" across Syrian and Iraqi territory was expected to be the subject of an urgent closed-door meeting of NATO allies on Tuesday.

U.S. officials have up to now declined to establish a no-fly zone along Turkey's border with Syria, fearing American war planes would be drawn deeper into Syria's four-year-old civil war if they patrolled the region with sufficient frequency to ground Syrian President Bashar Assad's air forces. The bloody conflict has pitted dozens of armed factions against Assad's hobbled government.

But the evolving shift in military posture toward ISIL in both Washington and Ankara appears likely to result in de facto aerial protection for ­Syrian rebel forces in the strategic border strip.

Downplaying the objective of U.S. air control of the envisioned safe zone isn't just semantics. Establishing an official no-fly zone would require a vote by the U.N. Security Council, where Russia or China could use their veto power to block it. Russia is Assad's sole European ally, and China routinely opposes any move by the world body that it interprets as getting involved in a country's domestic affairs.

The plan, which officials say is still weeks away from completion, centers around moderate Syrian rebels controlling a nearly 70-mile-long and 60-mile-deep buffer zone between the Euphrates River, at its eastern end, and the northern suburbs of Aleppo to the west. U.S. and Turkish warplanes would provide air support over the safe zone to stop the spillover of violence inflicted by ISIL, which has slaughtered thousands of ­perceived enemies with shocking brutality.

"Under consideration right now is deepening collaboration with our Turkish allies that's going to include air" strikes, said Pentagon spokesman Capt. Jeff Davis. "This will ensure greater security and stability along the Turkish border with Syria and help overall coalition efforts to defeat ISIL in both Syria and Iraq."

Turkish President Recep Tayyip Erdogan also had been reluctant to take more aggressive action until recently, for fear that driving out the militants would strengthen Kurdish forces battling the extremists and tempt them into alliance with Kurdish rebels in Turkey.