Obama, who recently asked the Pentagon and CIA for proposals to increase pressure in Syria, is considering sending 200 more members of U.S. special operations forces to advise and assist Kurdish and Arab militias seeking to close the so-called Manbij Gap, a porous 60-mile stretch that long has served as a cross-border corridor for the militants.
Islamic State forces holding the corridor have come under more coalition airstrikes in the last two weeks than any other target in Syria, according to U.S. officials. The Turkish military also has increased its artillery barrages against militants across its southern border.
Obama is considering giving more advanced artillery to Sunni Arab militias, increasing assistance to Syrian Kurdish forces, as well as making efforts to recruit more Sunni Arabs to fight alongside Kurds, according to officials who spoke on condition of anonymity in discussing ongoing planning.
The president -- who visited the CIA with Vice President Joe Biden, Secretary of State John F. Kerry and more than two dozen senior national security officials -- also may seek to resolve a simmering dispute between the CIA and the Pentagon over which Syrian rebel forces are best positioned to act as proxies and allies in the maelstrom of a multi-sided war.
Obama did not disclose his decisions after the nearly two-hour meeting, but he told several dozen agency employees in the CIA entrance hall that "we have momentum and we intend to keep that momentum."
He said that the ranks of Islamic State fighters have fallen to the lowest level in two years, that the group has not had a successful offensive in nearly a year, and that "we continue to take out their leaders, their commanders and those plotting terrorist attacks."
In recent months, the Pentagon has armed and helped advise Kurdish militias in northeastern Syria after an ambitious effort to train and arm a Sunni Arab rebel force collapsed last year. About 50 U.S. special operations personnel are based in the Kurdish-held zone.
Russia is known to support Kurdish militias arrayed in northwestern Syria. The Manbij Gap, which stretches from the outskirts of Aleppo to the Euphrates River, lies between the two Kurdish forces.
Although Syrian Kurdish militias are considered capable fighters, they are reluctant to push further south into territory held by Islamic State where few ethnic Kurds live.
U.S. intelligence officials have warned for months that funneling too much military and intelligence support to Syrian Kurds risks alienating the Sunni Arab militias that compete for territory with the Kurds, an ethnic minority.
It also could anger the government in Turkey, which hosts a major U.S. air base and is a crucial U.S. partner in the war.
Turkish Kurds have fought for greater autonomy for more than three decades. The government in Ankara fears that building Kurdish militias -- even in Syria -- into a stronger fighting force could lead to the formation of a breakaway state along the border.
During a visit to Washington this month for a nuclear summit, Turkey’s president,
Erdogan told Obama that U.S.-backed Kurdish militias moving west of the Euphrates River into the Manbij Gap would be a "red line" for his government, the official said. It's not known how Obama responded.
Turkey increased its shelling of Islamic State positions south of the border after a spate of terrorist attacks by the group, including a March suicide bombing on a high-end shopping street in Istanbul. Last week, U.S. airstrikes and Turkish artillery provided cover for rebel forces to assault the Syrian town of Rai, a border crossing at the western edge of the Manbij Gap.
Col. Steve Warren, a Baghdad-based spokesman for the coalition, said Wednesday that over the last two weeks U.S.- backed rebel forces have pushed Islamic State from more than a dozen small villages near the gap.
"While these operations don't encompass a lot of territory, this is critically important terrain for [Islamic State], because it is their last, best route to move people, money and supplies into Syria and Iraq," he said.
"There are some very ancient animosities through that region," Warren said. "Certainly, those have to be accounted for, certainly those will show themselves from time to time as different groups brush up against each other in the course of pursuing and fighting" the militants.
Jeffrey White, a former Defense Intelligence Agency official, cautioned that closing the Syrian border would hurt Islamic State but wouldn't be decisive in the war.
"This kind of anaconda strategy and slow attritional strategy, those are very long-term strategies, and [Islamic State] can offset them in various ways," he said. "The real issue is getting a force on the ground that can defeat them in battle. The Kurds don't look to be that force. They aren't large enough and they don't look to fight outside their traditional areas."
The U.S. and its allies long have sought to close the smuggling routes that lace Syria's northern border. The Sunni extremist group has drawn more than 36,500 foreign fighters to Syria and Iraq from around the globe, and many came through Turkey.
The latest coalition attacks also are aimed at cutting supply routes to Raqqah, the militants' self-declared capital in Syria. In Iraq, airstrikes and local ground forces already have cut off major roads leading west to Raqqah in an effort to steadily choke the Syrian city.
In February, U.S. special operations forces helped Kurdish fighters seize the northeastern Syrian town of Shadadi, severing what U.S. officials called a key Islamic State supply line.
The Pentagon has argued that the Kurds have proved to be the best allies on the ground in Syria. The Pentagon in October helped create a Kurdish-led rebel coalition called the Syrian Democratic Forces. It has received U.S. airdrops of weapons and supplies, and aid from special operations forces.
The Pentagon also is trying to build up Sunni tribes to fight alongside Kurds. It is taking Sunni Arab leaders out of Syria for up to two weeks of training in Turkey so they can help call in airstrikes, coordinate operations and arrange resupply from the U.S.-led coalition.
To meet that goal, the Pentagon last month relaunched a program to train and equip Syrian rebels, replacing the effort that collapsed in the fall after the first few hundred recruits were ambushed in Syria and handed over their U.S.-issued ammunition and trucks to an Al Qaeda affiliate.
A U.S. official, who was not authorized to speak publicly on the program, said the Pentagon aims to integrate more Sunni Arabs into the training to satisfy Turkey's demands and eventually to launch an attack on Raqqah.
"We know we can't take Raqqah with the Kurds alone," the official said. "That's why we keep pushing ahead with this training program -- getting indigenous forces on the ground is essential."