The Sunni uprising in Iraq has received enthusiastic support from many Gulf Arabs, despite official unease over the Islamic State in Iraq and the Levant, branded a terrorist group by governments in the region.

While public opinion is hard to gauge in the tightly-controlled Gulf monarchies, the lightning success of Sunni fighters in routing Iraqi government forces has been hailed with outpourings of vindication online and in private conversation.

The strong expressions of support suggest US allies like Saudi Arabia and the UAE may be pulled further from Washington, which backs the government in Baghdad.

"Today battle is waged in the Baghdad of Rasheed and Damascus of Waleed on behalf of the whole Islamic nation to restore anew its dignity. God, grant your victory," wrote Hakem al-Mutairi, head of a Kuwaiti movement of Sunnis from the austere Salafi school, on Twitter. He was referring to mediaeval rulers in the Syrian and Iraqi capitals, once the capitals of vast Sunni Muslim empires.

For many Sunnis in the Gulf, the collapse of Iraq's Shi'ite-led government forces heralds the decline of their greatest enemy, Iran. They regard Iraq's Prime Minister Nuri al-Maliki as an Iranian stooge.

"From Day 1 the Iraqi army wasn't real, it was a bunch of Shi'ite militias joined together. Sunnis are humiliated by Maliki and want to live a life of dignity so they decided to go into revolution," said Firas, an office worker in Riyadh.

Like other Saudis interviewed for this article, he withheld his family name for fear his comments would be seen by the authorities as supportive of ISIL, which Riyadh declared a terrorist organisation in March.

Under the terms of a February royal decree, any expression of support for the group in the kingdom is punishable by a long jail term. Social media users are highly aware that their feeds are scanned by the Interior Ministry.

Firas said support by Gulf Sunnis of the uprising against Maliki's government did not imply an endorsement of ISIL and its brutal tactics, which include the mass execution of prisoners.

"The Muslim people will not accept the ideology of ISIL because it's against our beliefs as moderates, which is what most Iraqis are," said Firas. "ISIL never had sympathy, even now. But still you have some people who justify their actions just to get rid of this sectarian government."

Gulf governments have walked a delicate line over Iraq, attacking Maliki and Iran as responsible for the violence and voicing staunch support for Sunni rights, while denouncing ISIL and other militant groups that proclaim jihad.

Riyadh suppressed a bloody insurgency a decade ago waged by al Qaeda members who had previously fought in Iraq and Afghanistan, and it fears the success of pro-jihad groups in Iraq and Syria will encourage extremism among its own citizens.

Thousands of Saudis and citizens of other Gulf countries are thought to be among the large foreign contingent fighting with ISIL and other militant groups in Syria and Iraq, spurring officials' fears of a domestic backlash.

Last month the Saudi Interior Ministry said it had uncovered a militant cell linked to both al Qaeda and ISIL that was planning attacks inside the kingdom.

However, years of state media criticism of the Iranian and Iraqi governments, coupled with the sometimes virulently sectarian comments by government-employed clerics, may have helped foster Sunni anger at Shi'ites.

The Gulf has long been a bastion of sectarian rivalry in the Middle East, with many Sunnis devoted to the Salafi and Wahhabi schools of Islam that regard Shi'ism as a deviant heresy. Many of the comments in recent days have contained outright gloating over Sunni victories.

"Do smile if you wish, the apostates (Shi'ites) in Kerbala are taking to the streets chanting 'We want the Messiah to appear' in order to save them from the holy warriors," wrote Ibraheem al-Faris, a professor of religion at Saudi Arabia's state-run King Saud University, on Twitter.

However, senior clergy in Saudi Arabia issued sermons on Friday that seemed aimed at discouraging jihad in Iraq. Its neighbour Bahrain too ordered religious leaders to avoid mentioning politics in the mosque.

The refusal of Sunni governments to openly support the Iraqi militants has prompted criticism by some Sunni citizens, who argue that their coreligionists are hobbled when compared to what they see as open Iranian backing of Shi'ites.

"Our war with the Shi'ites of Iraq has exploded forth. Their leaders have called out and their youth have answered. Their partisans arrive and their money moves. But helping the Sunnis has been banned (in the Gulf). Helping them is terrorism," was the Twitter comment of Saudi Sheikh Sowayan Shayah Hajri.

The two week-old Sunni advance in Iraq has certainly fuelled passions across the sectarian divide in the Middle East. The Iraqi government accuses Gulf states of stoking support for the militants by arming and backing Sunni fighters in neighbouring Syria. Last week, the Baghdad government even accused Saudi Arabia of promoting "genocide" in Iraq.

Riyadh says its support for Sunni militants in Syria does not extend to ISIL, which has fought against other Syrian rebel groups that Saudi Arabia arms and funds.

According to a social media website that analysed Twitter geolocational data, Saudis narrowly made up the biggest group of followers on one Arabic-language account affiliated with ISIL's propaganda wing, and were the second-largest group on another.

However it is difficult to draw conclusions from such data, given ISIL's illegal status in Saudi Arabia and its need to create new accounts as existing ones are closed. Saudis make up the biggest group of Middle Eastern users of Twitter generally.

Although ISIL has flown its black flag in the cities it seizes, its fighters are also supported by more moderate Iraqi Sunni tribes and politicians, who are angry at Baghdad's rule. Many in the Gulf say it is wrong to describe the Sunni uprising as an ISIL project.

In an opinion piece in the English-language Saudi daily Arab News entitled "Demonising the Sunni uprising in Iraq", Jordanian journalist Hani Hazaimeh wrote that the perception ISIL was leading the revolt played into Maliki's hands.

"What has been written so far in fact is doing al-Maliki's sectarian agenda a big favour," he wrote, arguing that the rebellion was a popular uprising against the Iraqi prime minister's "unjust and divisive policies".

To some Gulf Sunnis, the media focus on ISIL, and the alarm expressed by Western governments at events in Iraq, are an indication of double standards. They compare it to what they see as more muted criticism for Syrian President Bashar al-Assad, an Iranian ally.

"It shows the Western foreign policy is completely biased. Assad does whatever he wants to without any response to his war crimes, but as soon as a city falls in Iraq they want to do so much," said Mutaib, general manager of a company in Riyadh.-Reuters