Rebel fighters stand on the road between the villages of Zahraa and Nubol in northern Syria during clashes with pro-government fighters last month. (Mohamad Zeen/AFP/Getty Images)

The Obama administration is weighing the opening of a new front in the air war against the Islamic State in Syria, part of an offensive to push back militants along the western part of Syria’s border with Turkey and create a relatively safe zone for U.S.-backed Syrian rebel forces to move in.

Under the plan, U.S. aircraft flying from Turkey’s Incirlik air base would target positions the militants currently hold along the border north of Aleppo, eastward toward the besieged town of Kobane. Turkish special forces would move into the area to assist the targeting and help Syrian opposition fighters consolidate their hold on the territory.

President Obama, who has not yet approved the proposal, was briefed on its outline at a meeting with his senior national security advisers Wednesday.

The plan, which was developed over the past several weeks during extensive meetings between U.S. and Turkish diplomatic and military officials, also was a subject of discussion between Vice President Biden and Turkey’s top political leaders during Biden’s visit to Istanbul 10 days ago.

The proposal would at least partly address Turkey’s long-standing desire for a protected buffer zone inside Syria along the entire 511-mile border, while providing the faltering rebel fighters with a much-needed boost.

In exchange, U.S. access to ­Incirlik for the use of manned warplanes and armed drones throughout Syria would add as much as six hours to the time that individual strike aircraft could spend “on station,” locating and reaching targets. Aircraft currently striking Islamic State positions in northern and eastern Syria fly from bases in the Persian Gulf, a distance of about 1,000 miles.

“That access is huge,” a U.S. official said. At the same time, having Turkish special forces on the ground inside Syria would not only “breathe life into the Free Syrian Army” but also provide “more capable folks to help with targeting” for airstrikes.

Right now, the official said, targets are pinpointed with surveillance by unarmed aircraft flying from Incirlik and other bases in the region, as well as friendly Syrian “dudes with cellphones” on the ground. The official, who was not authorized to discuss the plan, described it on the condition of anonymity. A spokesman for the National Security Council declined to comment on the proposal or last week’s meeting with Obama.

If implemented, the plan would require significantly more U.S. resources than are now devoted to the fight against the Islamic State in Syria, including more planes and more money. Congress is debating both the funding and the new authorization for operations in Syria and Iraq that have already been approved by the president.

Although officials said the proposal is not intended to establish a traditional no-fly zone, requiring constant patrols against other aircraft entering the area — potentially up to 100 miles long and 20 miles deep inside Syria — its proponents recognize the potential for a “slippery slope” into a far more major operation.

Part of the administration’s risk assessment is whether Syrian President Bashar al-Assad will continue to allow overflights of his territory without activating Syrian air defenses, as he has with U.S. aircraft now striking the Islamic State in areas largely to the east of the proposed new front.

“Up to now, it’s been uncoordinated deconfliction,” the U.S. official said. “It’s not as though Centcom calls up the Syrians every morning and says, ‘Don’t go where we’re going.’ ” The U.S. Central Command is in charge of American military operations in Syria and Iraq.

Administration officials watched with concern last week as Syrian government aircraft bombed the north-central city of Raqqah, the center of Islamic State operations in the country. U.S. strikes have been targeting the militants in and around the city since September.

U.S. attacks in the proposed new corridor northeast of Aleppo would bring American aircraft into far more consistent proximity to Syrian aircraft, which regularly strike U.S.-backed rebel forces in that city. Top Pentagon officials have said that any attempt by Assad to interfere with U.S. aircraft would bring a broad attack on Syria’s air force and air defense system.

Officials described the new proposal, some elements of which were reported online Monday by the Wall Street Journal and Bloomberg News, as still in the planning stages. “There is nothing imminent and a lot of details still to come,” the U.S. official said.

Beyond the threat of direct conflict with Assad, the concept is fraught with additional risks and unknowns.

Turkish officials and retired Gen. John Allen, the administration’s chief envoy to the U.S.-led coalition against the Islamic State, have assessed that the Free Syrian Army can marshal sufficient trained forces to gradually move eastward into the zone. But the opposition’s track record is far from encouraging. In recent weeks, rebel fighters, including those who had received aid and training from the CIA, were pushed from their strongholds in Idlib province, west of Aleppo, by Jabhat al-Nusra militants allied with al-Qaeda.

The administration has authorized an expansion of the CIA program for rebels fighting in the northwest. Separately, the U.S. military is developing a training program for opposition fighters to move into defensive positions in areas of Syria that are already being targeted by airstrikes against the militants.

But even if trained rebels were up to the task, none of those areas, which constitute a third of the country in northern, central and eastern Syria, has yet been cleared of Islamic State forces. Militants surrounding Kobane appear to be holding their own despite weeks of steady U.S. bombardment.

Many officials, particularly in the White House and within the military, also remain distrustful of Turkey’s desire to draw the United States into a direct confrontation with Assad.

After months of resisting joining the U.S.-led coalition against the Islamic State, Turkey agreed in recent weeks to allow the establishment in its territory of a training base for Free Syrian Army fighters. Turkey is also training about 1,300 Iraqi Kurdish fighters, called peshmerga, to fight against the Islamic State.

The Turkish parliament has given President Recep Tayyip Erdogan permission to allow Turkish forces to enter Syria, but he has not used that authority. Although Turkey lets unarmed U.S. surveillance aircraft fly from Incirlik, it has refused until now to allow the facility to be used as a base for strike aircraft flying missions inside Syria.

Both Biden and Erdogan described their recent Istanbul meeting as a turning point toward closer cooperation between the NATO allies. But within days of Biden’s departure, Erdogan last week unleashed a broadside against what he called “foreigners” in the West who “don’t like us.”

“Foreigners love oil, gold, diamonds and the cheap labor force of the Islamic world. They like the conflicts, fights and quarrels of the Middle East,” he said in a speech in Istanbul, according to Turkey’s Hurriyet Daily News.

Calling for unity in the Islamic world, Erdogan said that “if we act together . . . it is possible to end the bloodshed in Iraq and killing of Syrian children.”