ISTANBUL—Jihadi veterans known collectively as the Khorasan group, which have been targeted in two waves of airstrikes by U.S. warplanes, are trying to broker an alarming merger between militant archrivals the Islamic State and Jabhat al Nusra, the official Syrian branch of al Qaeda.

The merger, if it comes off, would have major ramifications for the West. It would reshape an already complex battlefield in Syria, shift forces further against Western interests, and worsen the prospects for survival of the dwindling and squabbling bands of moderate rebels the U.S. is backing and is planning to train.

“Khorasan sees its role now as securing an end to the internal conflict between Islamic State and al Nusra,” says a senior rebel source. The first results are already being seen on the ground in northern Syria with a coordinated attack on two rebel militias favored by Washington.

All three of the groups involved in the merger talks—Khorasan, Islamic State (widely known as ISIS or ISIL), and al Nusra—originally were part of al Qaeda. Khorasan reportedly was dispatched to Syria originally to recruit Westerners from among the thousands of jihadi volunteers who could take their terror war back to Europe and the United States. But among ferocious ideologues, similar roots are no guarantee of mutual sympathy when schisms occur.

Current and former U.S. officials say they are unaware of any cooperation between ISIS and al Nusra, and they doubt that a merger or long-term association could be pulled off. “I find it hard to believe that al Nusra and Islamic State could sink their differences,” says a former senior administration official. “The rift between them is very deep,” he adds.

But senior Syrian opposition sources say efforts at a merger are very much under way and they blame Washington for creating the circumstances that make it possible. Moderate rebels accuse the Obama administration of fostering jihadi rapprochement by launching ill-conceived airstrikes on al Nusra while at the same time adamantly refusing to target the forces of Syrian President Bashar al-Assad in the U.S. military intervention in the region.

This, they say, has created the opening for a possible understanding between the jihadists and is creating sympathy for al Nusra. Other Islamist rebels and the wider population in insurgent-held areas in northern Syria question American motives and designs and remain furious at the U.S. decision not to help topple Assad.

“Al Nusra knows more airstrikes are coming, so why wait,” says an opposition source. If the Americans are going to lump them together with ISIS, maybe best to join forces. “What made the possibility of their coming together are the airstrikes.”

The opposition sources, who agreed to interviews on the condition they not be identified, warn that mounting cooperation between the two jihadist groups already is evident in specific operations.

Earlier this month, ISIS sent more than a hundred fighters in a 22-vehicle column to assist its onetime competitor, al Nusra, in the final assault on a moderate Islamist rebel alliance, the Syrian Revolutionaries Front, commanded by Jamal Maarouf in Idlib province.

The jihadis also targeted a secular brigade of insurgents, Harakat al-Hazm, which the U.S. has supplied with advanced anti-tank weaponry, because it tried to intervene and separate the SRF and al Nusra.

“Da’esh fighters weren’t really needed,” says one of the sources, “Al Nusra had sufficient numbers but the support given is highly symbolic.” (Da’esh is the Arabic acronym for ISIS.)

The coordination being claimed between the two groups would be the first time ISIS militants have cooperated with al Nusra since the winter ,when al Qaeda’s overall leader Ayman al-Zawahiri issued what seemed a definitive statement: “Al Qaeda announces that it does not link itself with [ISIS] ... It is not a branch of the al Qaeda group, does not have an organizational relationship with it.”

The al Qaeda old guard and the ambitious ISIS leader Abu Bakr al-Baghdadi, who bristled at orders from Zawahiri, fell out over strategy and the attacks that his mainly foreign fighters were mounting against Syrian rebels. But the rift was, not least, a matter of personalities and egos. Al-Baghdadi has since attempted to declare himself the true leader of all true Muslims (by his lights) as the Caliph of the Islamic State. Zawahiri is not about to sign on to that.

Thus reports that al-Baghdadi may have been badly wounded or even killed in a U.S.-led coalition airstrike mounted last week near Mosul, while they may sound like good news for the coalition, could be even better news for the jihadis. Syrian rebel sources say al-Baghdadi’s elimination might well assist an agreement being struck between ISI and al Nusra.

The senior opposition sources say the coordination in the fight with the Syrian Revolutionaries Front was agreed on at a meeting held just west of Aleppo between representatives of the two jihadi groups and overseen by members of the Khorasan group.

U.S. intelligence agencies accuse the Khorasan veterans of plotting attacks against commercial airliners in the West. The U.S. targeted them with a wave of sea-launched cruise missiles on Sept. 23 and last week hit again with wide-ranging airstrikes on al Nusra positions as well, partly in a bid to hit the veterans. Several members of the group have been killed, but top leaders are still thought to have escaped the targeting and U.S. officials say they can’t confirm who has survived and who hasn’t.

There were representatives at the meeting from other hardline groups as well, such as Jund al-Aqsa, a jihadi offshoot, and Ahrar al-Sham, a group al Qaeda was instrumental in forming.

At the meeting a few nights before the final jihadi push against the SRF, which was attended by al Nusra leader Abu Mohammad al-Golani, the participants agreed, say opposition sources, that the Syrian Revolutionaries Front should be eliminated as an effective fighting force.

The assault on the weekend of Nov. 1 sealed weeks of battles between al Nusra and the SRF. The jihadis have now captured a series of towns and villages in Idlib province—Maarshorin, Maasaran, Dadikh, Kafr Battikh, Kafr Ruma, Khan al-Subul, and Deir Sunbul, Maarouf’s hometown. And al Nusra fighters have in recent days moved further north, coming within three miles of the important crossing on the Turkish border at Bab al-Hawa. The SRF has been left with virtually no territory.

Meanwhile, the secular Hazm movement was forced by al Nusra fighters to withdraw from its strongholds in Idlib, including Khan al-Subul, where it stored about 10 percent of its equipment. Hazm denies reports that jihad fighters managed to seize U.S.-supplied TOW anti-tank missiles, but concedes that al Nusra was able to secure 20 tanks, five of which were fully functional, six new armored personnel carriers recently supplied from overseas, and dozens of the group’s walkie-talkies, with the result that Hazm fighters elsewhere had to ditch their sets lest ISIS listen in.

(Some Hazm members bought the walkie-talkies themselves from Best Buy during a visit to the U.S.—suggesting that aside from TOW missiles the Obama administration has not been that generous in supplying the brigade.)

Hazm, which has now withdrawn nearly all of its approximately 4,000 fighters to Aleppo, remains shaken not only by al Nusra’s offensive, but also by the defection to the jihadis of 65 of the 180 fighters it had stationed in Khan al-Subul, a third of the force based there.

Rebel commanders in other Free Syrian Army militias acknowledge that they, too, are worried by defections among their ranks to al Nusra and the Islamic State and fear the prospects of more as Assad’s forces further squeeze moderate rebels from one direction and the jihadis and hardline Islamists from another.

The picture for moderate rebels remains increasingly bleak. Western-backed FSA brigades in northern Syria are now holed up in three pockets: in Aleppo, and to the west and north of the city.

To the northeast, FSA brigades are preparing to retreat from a string of towns including Marea and Tell Rifaat. “We have no choice but to make a strategic withdrawal,” says rebel commander Abdul Rahman of the 3,000-strong Jaysh al-Mujahedeen or Army of Mujahedeen. “We can’t hold off Da’esh.”

In Aleppo, Hazm has signed a non-aggression pact with al Nusra—forced to do so after a checkpoint the moderates came under sustained jihadi attack. Few moderate rebels remain confident that al Nusra will hold off attacking other moderate insurgents, arguing they are doing exactly what they criticized Da’esh for doing—fighting the rebels rather than focusing exclusively on Assad.

“If this continues,” says a dismayed rebel source, “there will be no FSA brigades left for the U.S. to train. Assad must be grinning.”

With additional reporting by Shane Harris