BEIRUT Al Qaeda's affiliate in Syria, the Nusra Front, is facing mounting pressure from its own members to reconcile with its rival Islamic State and confront a common enemy after U.S.-led air strikes hit both groups this week.
But that move would require pledging loyalty to Islamic State, which has declared a caliphate in territory it controls in Iraq and Syria, which would effectively put an end to the Nusra Front, fighters in the group say.
Nusra, long one of the most effective forces fighting Syrian President Bashar al-Assad, was weakened this year by battles with Islamic State, an al Qaeda splinter group that routinely employs ruthless methods such as beheadings and mass executions.
The two share the same ideology and rigid Islamic beliefs, but fell out during a power struggle that pitted Islamic State head Abu Bakr al-Baghdadi against al Qaeda chief Ayman Zawahri and Nusra Front leader Abu Mohammad al-Golani.
But U.S.-led air and missile strikes, which have hit Nusra as well as Islamic State bases in Syria, have angered many Nusra members who say the West and its allies have joined forces in a "crusader" campaign against Islam.
Sources close to Islamic State said some Nusra fighters were joining them after the strikes and there was a growing sense among many that it was time to put their differences aside.
"There are hardline voices inside Nusra who are pushing for reconciliation with Islamic State," a source close to Nusra's leadership told Reuters, though he doubted it would happen.
"I know Golani. He would never reconcile with Islamic State. If he ever does it, it would be in a direct order from the leadership, and that is Zawahri himself."
However, one Islamic State fighter said he believed there was an "80 percent chance that the brothers of Nusra will join the State".
The Syrian Observatory for Human Rights, a Britain-based monitoring group, said on Friday over 200 fighters had joined Islamic State in the northern Aleppo area, many from the Nusra Front, since U.S. President Barack Obama said he was prepared to strike the group in Syria.
In an audio message posted on militant forums on Thursday, a senior al Qaeda figure warned Muslims against joining Islamic State and called on fighters in Syria to "rescue the ship of jihad, and reach it before it deviates from its course."
The message from Muhammad bin Mahmoud Rabie al-Bahtiyti told fighters in Syria to avoid infighting and fanaticism, the SITE monitoring service said.
He said they should build a caliphate avoiding "oppression, infidel-branding the Muslims, killing the monotheists and dispersing the rank of the mujahideen," SITE said.
Even before the air strikes, Nusra was facing difficulties and was losing fighters to Islamic State, which is seen as more organized and determined to impose Islamic rule.
"This goes without saying, this is a crusader war that includes all infidel nations against the Islamic State," said Nusra commander Abu Mussab al-Makdessi in a voice message posted in jihadi forums online in response to a question from an Islamist about the group's reaction to the strikes.
"Regardless of what happened between us, they remain our brothers, and the ideological bond between us is stronger than anything. We are ready to fight by their side ... our blood is their blood."
One former Nusra fighter inside Syria said the air strikes had strengthened Islamic State's position even further.
"Nusra is in a very difficult situation. I think now it should just announce the end of itself. Zawahri has to be brave," he told Reuters.
"It is no longer like the old days. He needs to understand this. This is a new era with a caliph," he said.
Yet sources from and close to both sides said it would be difficult for the two groups to work together without merging - and with Nusra in a weaker position, that would effectively mean being subsumed by Islamic State.
Such a decision should be taken by Zawahri himself. Sources say he should give a speech outlining al Qaeda's position on the attacks soon.
"Golani does not trust Baghdadi, and he doesn't like his politics and agenda, he sees it as distorted and astray," the source close to the Nusra leadership said.
But there were figures inside Nusra who were seen as more extreme that wanted to make peace with Islamic State, he said, naming top Nusra cleric Sheikh Sami al-Aridi, who is also close to Golani, as a particularly strong proponent of reconciliation.
If that does happen, it would be an alliance and not a merger, the source said, adding merger would be impossible since Baghdadi, who has declared himself leader of all the world's Muslims, would want Zawahri to pledge allegiance to him.
But even if Zawahri only orders Nusra to fight alongside Islamic State, it is likely to accelerate any hemorrhaging of influence from him to Baghdadi.
Nusra, which has been trying with allies to remove its name from the U.N. terrorist list, was taken by surprise when U.S.-led coalition warplanes bombed several of its positions in Idlib province.
Several commanders are believed to have been killed in the strikes, including Kuwaiti-born Mohsin al-Fadhli - also known as Abu Asmaa al-Jazrawi - reputedly a former member of Osama bin Laden's inner circle, and whom United State officials call the head of the "Khorasan group".
Khorasan is the Islamic term for an area including parts of Pakistan and Afghanistan, where al Qaeda's main council is believed to be in hiding.
ATTACKS ON LEADERS
The source close to the Nusra leadership said the Khorasan group was led by veterans from Afghanistan.
"It is a very small group - dozens of fighters only. It is more symbolic because it is composed of veterans who came from Afghanistan, and they are all wanted by Washington. They directly follow the Qaeda leadership," he said.
Another Nusra source said Fadhli and al-Golani had fallen out recently but had continued to cooperate.
The strikes on the "Khorasan group" also showed that the United States knew more than it should, he said. This could embarrass Nusra leaders by leading fighters to suspect they were infiltrated. Nusra leaders have reportedly gone underground and changed locations since the strikes.
"The precision of the raids on these positions shows clearly that the Americans have intelligence members among the Nusra fighters. It is very clear to us now," the Nusra source said.
Islamic State fighters and sympathizers have vowed to respond to the strikes. In particular they accused Saudi Arabia of being the mastermind behind the attacks.
But jihadis also say the group is in no hurry and is waiting to see what the coalition plans to do. The group's leaders went underground even before the strikes, it has evacuated most of its buildings in its stronghold of Raqqa province, and its fighters are rarely seen on the streets.
"The response will be in every country that took part in bombing Muslims and their state," said an Islamic State fighter in Syria.
"Masks have fallen. In our eyes, it fell a long time ago, but now Muslims across the world saw it."
(Editing by Alexander Dziadosz and Will Waterman)