This photo was posted on the Twitter page of al-Qaeda’s affiliate in Syria, Jabhat al-Nusra, on April 1. It shows fighters marching toward the northern village of al-Ais in Syria’s Aleppo province. (Uncredited/AP)

The Obama administration has begun to see Jabhat al-Nusra, ­al-Qaeda’s affiliate in Syria, as a global threat that could eventually ­rival the Islamic State, echoing a Russian argument that it has long resisted.

A new U.S. proposal to coordinate counterterrorism operations in Syria with Russia, discussed by President Obama last week with President Vladi­mir Putin, is partly designed to stop Moscow’s Syrian government ally from bombing ­civilians and U.S.-backed moderate opposition ­forces.

But stopping al-Nusra, which has been one of the main beneficiaries of the ongoing Syrian civil conflict, appears to have become an almost equally important goal. The proposed deal would start with coordinated U.S. and Russian strikes against al-Nusra.

The group’s capacity is growing, and it is “the largest al-Qaeda affiliate now in the world,” Brett McGurk, the administration’s envoy to the global coalition against the Islamic State, said this week.

Nicholas Rasmussen, director of the National Counterterrorism Center, told Congress last week that senior al-Qaeda leaders are increasingly migrating to a “growing safe haven in Syria.” Some have come from Pakistan and Yemen, where the group has suffered ­losses, while others may be among those recently released from years of detention in Iran.

“These leaders include individuals who have been part of the group since the time even before 9/11,” Rasmussen said. “And now that many of them are in Syria, we believe they will work to threaten the U.S. and our allies.”

The operatives are believed to include those involved in al-
Qaeda’s “external operations” directorate. Rasmussen did not provide names, but there are strong indications that one of them is Saif al-Adel, an Egyptian who worked closely with Osama bin Laden and once served as al-Qaeda’s military commander. Adel, who fled from Afghanistan to Iran in 2001, was released from a form of house arrest by that country last year in exchange for an Iranian diplomat being held hostage in Yemen.

Others freed by Iran include Abu Kayr al Masri, who once ran al-Qaeda’s management council, and Abdullah Ahmed Abdullah, an Egyptian referred to in a 2008 classified U.S. document as the “most experienced and capable operational planner not in U.S. custody.”

“It’s hard to gauge just how much it will benefit them,” Seth Jones, director of the International Security and Defense Policy Center at the Rand Corp., said of al-Qaeda’s dispatch of senior operatives to al-Nusra. But “al-Qaeda is putting all its chips in Syria.”

The U.S. proposal to Russia has been criticized by some U.S. diplomatic, intelligence and military officials as naive, given Moscow’s repeated violations of agreements with the West. The administration had long seen al-Nusra as a relatively marginal threat to this country, and it remains a “sideshow” to the main fight against the Islamic State, one U.S. official said.

But a senior U.S. intelligence official disagreed. While all attention is focused on the Islamic State, which has shifted much of its own focus toward terrorist attacks in Europe and the United States as it has lost territory in Syria and Iraq, this official said, “it’s only a matter of time before we get hit by core al-Qaeda. It’s a reality.”

The officials spoke on the condition of anonymity to discuss the sensitive issue.

Just as the Islamic State has done with its own potential recruits, al-Qaeda leader Ayman al-Zawahiri in September called on Muslims in the United States to carry out lone-wolf attacks and to consult the organization’s Inspire magazine for ideas.

Earlier this month, one of bin Laden’s sons, Hamza, threatened revenge for the U.S. killing of his father in Pakistan in 2011. “If you think that your sinful crime . . . has passed without punishment,” he said, “then you thought wrong. What is correct is coming to you, and its punishment is severe.”

The United States has centered on the Islamic State since its forces­ flooded across Syria and Iraq in the summer of 2014. Born from the remnants of al-Qaeda’s Iraq operations in the previous decade, it competed with the Syrian affiliate, al-Nusra, whose leadership eventually broke relations between the two over doctrinal disputes in 2013.

Since then, they have taken sharply different paths in Syria. While the Islamic State, from its headquarters in the north-central city of Raqqa, seized territory to the north and across the Iraqi border to the east, al-Nusra jumped directly into the civil war against Syrian President Bashar al-Assad.

Early in its bombing campaign that began in Iraq and Syria in September 2014, the U.S. military launched an airstrike against a group of senior al-Qaeda members, known as the “Khorasan Group,” who had relocated to Syria and were, according to U.S. officials, planning “imminent” attacks against the United States.

In addition to destroying training camps, the strike killed Muhsin al-Fadhli, a notorious al-Qaeda financier and facilitator. The military described Fadhli, who had been released by Iran, as the Khorasan leader and “among the few trusted al-Qaeda leaders that received advanced notification of the Sept. 11, 2001, attacks.”

After Fadhli’s death, FBI Director James B. Comey said the Khorasan Group had been diminished and the Islamic State posed the greater threat. “ISIL is not your parents’ al-Qaeda,” Comey told CNN last summer. “It’s currently the threat that we’re worrying about in the homeland most of all.” ISIL, ISIS and Daesh are alternate names for the Islamic State.

But over the course of 2015, al-Nusra gradually grew in strength and stature, becoming the most powerful, and successful, anti-Assad force; it is currently believed to number about 10,000 fighters. Al-Nusra began to win even as so-called moderate opposition forces­ lost ground in populated areas of western Syria, where they were starved of resources by U.S. and regional backers who feared they would turn weapons over to the militants.

Al-Nusra’s successes were a major factor in Russia’s own bombing campaign, begun in September in support of Assad. When the administration charged that the Russian and Syrian air forces were using al-Nusra’s presence as a smokescreen to target the moderate opposition, Moscow said that the al-Qaeda fighters were interwoven with the rebels and that it was up to the United States and its allies to separate them.

In February, Secretary of State John F. Kerry and his Russian counterpart, Sergei Lavrov, formed an international group that pushed for a cease-fire in the Syrian civil war, with only the Islamic State and al-Nusra excluded.

When Russia initially paused its bombing under the cease-fire, the United States took advantage of the suddenly clear skies in western Syria to attack al-Nusra in April, targeting its spokesman, Abu Firas al-Suri, and a car carrying what Pentagon officials said were “core al-Qaeda” members.

Within weeks, the truce began to break down, with Russian and Syrian aircraft and artillery pummeling opposition ­forces, and civilians, in the name of fighting al-Nusra. The United States, Moscow repeatedly charged, had not lived up to its agreement to separate the opposition forces­ it backs from the militants.

In the proposal Kerry took to Moscow last week, the administration said that if Russia would use its leverage to ground the Syrian air force, the United States would work harder to end the overlap. And it would share intelligence and join the Russians in bombing al-Nusra.