But sources say it may not work without weaker Al Nusra merging with ISIS

Demand for alliance comes after both groups were

Al Qaeda's affiliate in Syria is facing mounting pressure from its members to form an ultra-alliance with the rival Islamic State to confront a common enemy after U.S.-led air strikes hit both groups this week.

Al 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.

U.S.-led air and missile strikes, which have hit Al Nusra as well as Islamic State bases in Syria, have angered many Al Nusra members who say the West and its allies have joined forces in a 'crusader' campaign against Islam.

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 Al Nusra Front leader Abu Mohammad al-Golani.

But merging 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 Al Nusra Front, fighters in the group say.

Rising up: Supporters of the Al Nusra Front protest in Aleppo against Syrian President Bashar Al Assad and the U.S.- led international coalition after the rebel group was hit by air strikes this week

Members of the Al Nusra Front (above) want their leaders to reconcile with its rival Islamic State to confront a common enemy, namely President Assad and Western forces launching bombing raids in Syria and Iraq

Sources close to Islamic State said some Al 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 per cent 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.

Sources close to Islamic State (above) said some Al Nusra fighters were joining them after the U.S.-led strikes

The two groups share the same ideology, but fell out during a power struggle that pitted ISIS head Abu Bakr al-Baghdadi (above) against Al Qaeda chief Ayman Zawahri and Al Nusra Front leader Abu Mohammad al-Golani

Even before the air strikes, Nusra was facing difficulties and was losing fighters to the Islamic State, which is seen as more organised 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.

Structure: Abu Bakr al-Baghdadi employs a network of deputies, councils and executives with the power to act on the ISIS leader's behalf. These senior militants are believed to give orders to a group of 24 regional governors, who in turn head 'councils' managing everything from the sale of oil, to the treatment of prisoners

'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 haemorrhaging 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.

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 sympathisers 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.