The Obama administration is pressing Turkey to deploy thousands of additional troops along its border with Syria to cordon off a 60 - mile stretch of frontier that U. S. officials say is used by Islamic State to move foreign fighters in and out of the war zone. The U.S. hasn’t officially requested a specific number of soldiers. Pentagon officials estimated that it could take as many as 30,000 to seal the border on the Turkish side for a broader humanitarian mission. Cordoning off just one section alone could take 10,000 or more, one official estimated. It’s unclear how Turkey will respond. Turkish officials said they agree that tighter border control is necessary, and have begun implementing some measures. They suggested that the Pentagon troop estimates are inflated, but declined to give a number of their own. In return for doing more to fight Islamic State, Turkey is seeking more financial help from Europe to deal with its 2.2 million Syrian refugees, as well as support for a safe zone in Syria—an idea that has been shelved by the Obama administration as too risky and complicated. U.S. officials say a bigger Turkish border deployment—including infantry and artillery units—would be the most realistic way to close off key transit routes on which Islamic State fighters in Syria rely and stem the flow of foreign fighters into Europe. U.S. and Turkish leaders agreed in principle in July on a joint effort for the border, but detailed operational planning has been slowed by discord—between the two governments as well as within the Obama administration. The U.S. is prepared to launch airstrikes to support Syrian militants fighting Islamic State south of the border, but the plan has been slowed by the Russian airstrikes and a chaotic battlefield. The proposed joint operation has taken on added urgency since the Nov. 13 terrorist attacks in Paris. This week’s shoot down by Turkey of a Russian military jet added new complications, but U.S. officials said it didn’t change what they saw as an urgent need for Turkey to substantially increase the density of Turkish forces along that frontier, particularly between two border towns that U.S. intelligence agencies believe Islamic State has been using as transit points. “The game has changed. Enough is enough. The border needs to be sealed,” a senior Obama administration official said of Washington’s message to Ankara. “This is an international threat, and it’s all coming out of Syria and it’s coming through Turkish territory.” Turkish officials say they are willing to take the necessary steps, provided the U.S. takes steps as well. “Turkey is determined to clean Daesh from the 98 kilometers of border between Kilis and Jarabulus,” said a senior Turkish government official, referring to Islamic State by an Arabic acronym. “There is no need to receive any kind of warning or advice from anyone, including our U.S. partners.” In conjunction with the deployment of Turkish military forces on the Turkish side of the border, the U.S. agreed in principle to provide air support for a coalition of rebel forces to fight Islamic State on the Syrian side. Turkish officials see those ground operations inside Syria, with U.S. support, as the priority. “When the operation starts and local forces are making progress, it is obvious to increase troops on our border,” said the senior Turkish government official. Top leaders at the Pentagon, however, are skeptical that Turkey can muster rebels in sufficient numbers in northern Syria to fight Islamic State rather than the Assad regime, particularly since Russia intervened with air power in support of Damascus. Obama administration officials say Ankara should take the initiative by cordoning off the border on the Turkish side, and accept that the ground operation inside Syria will build up more gradually, as enough rebels become available. Discord over establishing a protected zone on the Syrian side of the border has been overshadowing progress on a border plan. “Just closing down the border would not be enough to solve our problems and would not address Turkey’s demands,” a Turkish official said. The White House and Pentagon have long opposed creation of a formal safe zone, arguing it would draw the U.S. deeper into the war. U.S. officials, including the Pentagon and the State Department, conducted an assessment this fall of how many troops it would take to create a safe zone, and concluded that it would take about 30,000 troops. Officials used that figure as a reference point to estimate the needs for a cordon, but said that could turn out to require fewer troops. U.S. officials have warned Ankara that it could face “significant blowback” from European powers if Turkey fails to close the border and if foreign fighters succeed again in slipping out of Syria and conducting terrorist attacks in Europe. “This is really a question of their border security,” a senior U.S. official said of Turkish authorities. “They need to step up their game when it comes to this and they can’t necessarily look to us to fortify their border for them. Paris is a wake-up call to them that this is a problem they don’t have under control.” Since the Paris attacks, pressure has been mounting on the U.S. and its allies to step up the fight against Islamic State in Syria and Iraq, as well as on Turkey to curtail the flow of foreign fighters. When they met days after the Paris attacks this month on the sidelines of a Group of 20 leading nations summit, President Barack Obama told Turkish President Recep Tayyip Erdogan that Turkey needed to move quickly to cordon off the border area in order to reduce the risk of more terrorist attacks in Europe and beyond, according to U.S. and Turkish officials. In the meeting, Erdogan told Obama he agreed on the need to prevent Islamic State from transiting the Turkish border, though he offered few specifics, according to U.S. officials. A Turkish official said the Turkish army could begin beefed-up deployments along the border shortly, though he wouldn’t provide numbers. Some of the militants who took part in the Paris attacks and other recent terror plots passed through the 60-mile stretch of border, according to U.S. and European officials. Under the joint operation that U.S. and Turkish leaders agreed in principle in July, the ground operation on Syrian soil against Islamic State militants would be spearheaded by Turkish-backed rebel groups made up of Syrian Arab and Turkmen fighters, according to U.S. and Turkish officials. But U.S. officials say many of the rebel groups whom Turkey and the U.S. were counting on to take part in the ground offensive have moved south to help defend the rebel stronghold of Aleppo against the Assad regime’s new Russia-backed offensive. The Turkmen forces also have come under pressure by Russian airstrikes that have displaced thousands of Turkmen civilians in recent weeks. Mr. Erdogan warned Russia to stop the attacks in the days leading up to the Turkish military shootdown of a Russian warplane along the Syrian border. “There’s no local, capable, motivated force that is prepared to clear this area at this time,” a senior defense official said of the Pentagon’s thinking. “There are two sides to every border. If Turkey is motivated to seal their border, there is nothing that’s stopping them from using their conventional forces to do so,” the senior defense official said. U.S. officials said Turkey has taken some steps in recent months to slow the flow of foreign fighters, including closing the main border-crossing points along the country’s frontier with Syria. U.S. officials credit Turkish authorities, for example, with arresting key Islamic State militants, in some cases at their own initiative. “This is not the Turkey of six months ago. Turkey has shifted,” the senior Obama administration official said. The U.S. is particularly urging Turkey to deploy troops to cordon off border areas around and in between the border towns of Jarabulus and Cobanbey, which U.S. officials say Islamic State has increasingly used to shuttle foreign fighters and supplies between Syria and Turkey and beyond. U.S. officials point to the success of Syrian Kurdish forces cordoning off a stretch of the Syria’s border with Turkey from the eastern banks of the Euphrates River to Iraq. The operations conducted by the Kurds have almost completely halted the flow of foreign fighters there, they said. Before Paris, Turkish officials often rebuffed U.S. calls for a larger border force, saying the frontier was simply too long to effectively seal, no matter how many soldiers were deployed. Turkish officials pointed to Washington’s inability to seal off the U.S.-Mexico border as an example of how difficult such operations can be. U.S. officials chafe at the comparison. “If we were at war with Mexico, we’d close that border,” a senior administration official said of Washington’s response.