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 US 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.
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