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 Russian 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 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.”