In agreeing to cooperate to clear Islamic State forces out of a 60-mile-long strip of northern Syria along the Turkish border, the United States and Turkey have taken a major step toward increasing pressure on the militant group and easing their differences on the Syrian conflict.
The Obama administration, whose top priority is battling the Islamic State, also known as ISIS or ISIL, would get to use Turkish air bases to attack the militants on a new front and has won a new commitment from Turkey to try to shut off some of the group’s most important supply lines.
Turkey, whose primary goal has long been to oust President Bashar al-Assad of Syria, would get a new degree of security along its border — and in the process, keep a Syrian-based Kurdish militia force that it considers a threat from making inroads to the area.
But when it comes to carrying out the agreement, which was reached over the weekend and was described by four senior American officials, significant complications remain.
Credit Murad Sezer/Reuters
Not least, the new campaign draws the United States more deeply into the chaotic Syrian conflict, which the Obama administration had been determined to resist. The United States has yet to disclose which Syrian insurgent forces it will enlist in the effort, and the deal shunts aside the Syrian Kurdish Y.P.G. militias that have lately been the United States’ main partners in fighting the Islamic State in Syria, but whom Turkey considers enemies.
The agreement, reached after months of discussions, envisions that intense bombardment by American and Turkish forces will help Syrian forces that the United States considers relatively moderate to grab territory from the Islamic State.
Though the depth of the new zone, along roughly 60 miles of border starting near the Syrian city of Aleppo and stretching to the Euphrates River, has not yet been agreed on, it could include important Islamic State strongholds like Dabiq and Manbij, and officials said it was considered crucial to the group’s smuggling efforts. It could also include the Islamic State-held town of Al Bab, which has been routinely bombarded by government helicopters.
But Turkey’s history of being less than consistent and enthusiastic in stopping the Islamic State fighters who have streamed into Syria through its borders has left some analysts debating whether the government of President Recep Tayyip Erdogan is more focused on fighting Kurdish militant groups — and a rising domestic political opposition — than on stopping the Islamic State.
Turkish and American officials do not even describe the plan the same way. Both countries call the proposed area of operations an “Islamic State-free zone.”
The American officials insist that a no-fly zone that would keep Syrian government forces from bombing there is not part of the deal and say they have not discussed formally designating a “safe zone” for returning refugees. But they do note that Syrian aircraft have tended to avoid areas farther east where the United States is regularly flying airstrike missions against the Islamic State.
That dual terminology offers a flash of hope in some quarters that the border strip might be the first step toward a broader intervention — especially among displaced Syrians and for Mr. Assad’s Syrian opponents, armed and civilian, many of whom have long pleaded for international air power to stop devastating government airstrikes against insurgent-held areas.
But more immediately, the deal suggests that for now, in one important place on the Syrian battlefield, there is at last some degree of convergence between the goals of those who believe the primary target is Mr. Assad, and those who believe it is the Islamic State.
The “ISIS-first” camp includes President Obama and others who worry that if Mr. Assad falls first, the Islamic State or other Islamist extremist groups could take over the country.
The “Assad-first” camp includes Turkey and a vocal group of dissenters, including some State Department officials and former Obama administration officials. They argue that the Islamic State arose in part because of anemic international aid early in the uprising to Syrian insurgents led by army defectors, before foreign fighters streamed in.
American airstrikes had focused on helping Kurdish militias fight the Islamic State in the far eastern part of Syria and in Iraq, where the group is concentrated away from battles between Mr. Assad’s government and the rest of the insurgency, precisely to avoid mixing the two missions.
Now, though, the potential to use Turkish bases to stage more intense airstrikes against additional Islamic State strongholds and supply lines was seen by some analysts as an important opportunity — though with caveats.
“To the extent it excludes ISIL from the border area and from a certain zone within Syria, it is positive,” said Frederic C. Hof, who was an adviser to former Secretary of State Hillary Rodham Clinton on Syria and favors more robust intervention. “At least in principle, it creates an area where people who are trained and equipped in Turkey can be inserted in Syria without opposition, where they can join up with existing units who are fighting ISIL.”
He added: “The question for me is whether there is the potential for this to become more than a small protective zone in a relatively remote part of Syria.”
Another question is which Syrian insurgent groups would most stand to benefit, as those that the United States has vetted through a publicly declared Pentagon training program are far too few — there are only 60. Other insurgents who have been trained covertly by the Central Intelligence Agency, while more numerous, are enmeshed with or fighting alongside more hard-line Islamist groups, including the Nusra Front, Al Qaeda’s Syria affiliate.
In any case, a new American air campaign along the border would entail a far higher degree of coordination with Syrian insurgents than the United States has yet undertaken, including, American officials said, working out a system for using targeted airstrikes.
And in a sign that the United States plans to continue diplomatic engagement with some Syrian elements, Secretary of State John Kerry said on Monday that he had appointed a veteran foreign service officer to serve as the envoy to the Syrian opposition: Michael Ratney, the former United States consul general in Jerusalem.
One area of concern about the new Turkish engagement against the Islamic State, which started with airstrikes on Thursday, is that Turkish forces seemed to be at least as interested in waging a new campaign against Kurdish militants. Almost immediately, Turkish forces began striking at elements of the P.K.K. militant group, which is listed as a terrorist organization by Western countries as well, in Iraq’s Kurdistan region. The attacks, which Turkish officials said followed attacks by the group on police forces, signaled the end of a two-year cease-fire with the group that had helped keep a relative peace in Kurdish areas of southern Turkey.
Blaise Misztal, the national security director at the Bipartisan Policy Center in Washington, said that even as American officials won Turkish commitments to fight the Islamic State, they also gave Mr. Erdogan a pretext “to unleash a conflict that could tear apart Turkey and its neighbors.”
The Kurdish Y.P.G. militias, the P.K.K.’s affiliate in Syria, are also seen as a threat by Turkey. But those fighters have been a crucial fighting force against the Islamic State in eastern Syria, and they took advantage of American airstrikes to fight off an assault on the town of Kobani that lasted for months.
The deal between the United States and Turkey, as described by the American officials, would implicitly freeze the Y.P.G. from making inroads into the border area near Aleppo. And on Monday, Y.P.G. fighters accused Turkey of going further, saying that its militia forces had been attacked by Turkish strikes in an area they had just managed to take from the Islamic State.
Such an attack would have been a new escalation, but Turkish officials later suggested they had not been purposely targeted. They also said they would not see the group as an enemy if it did not harm Turkey.
Another senior American official said that the Islamic State had risen as a relatively more urgent threat to Turkey after the Islamic State attacked new border areas and especially after it claimed responsibility last week for its first deadly bombing inside Turkey, which killed 32 young Kurdish activists. At the same time, the semiautonomous government of Iraq’s Kurdistan region, a crucial American ally in the region, said that it welcomed the new American and Turkish cooperation.
Falah Mustafa Bakir, the head of Iraqi Kurdistan’s foreign affairs department, said in an interview in Washington that the added threat to the Islamic State was a benefit for the entire coalition against the group, including the Kurdish government in Iraq.
“ISIS should be targeted in both Iraq and Syria,” he added. “If ISIS continues to have a safe haven in Syria, you will have to deal with that problem over again.”