BEIRUT (Reuters) - Islamic State (IS) insurgents in Iraq and Syria have issued a pamphlet calling for Muslims to pledge loyalty to their chief, Abu Bakr al-Baghdadi, pressing a campaign to establish a "caliphate" even as U.S.-led forces bomb their positions.
Baghdadi's claim to the mantle of Islam’s caliphate, the Muslim state established by the Prophet Mohammad’s first followers after his death, has infuriated many Islamic leaders and ignited an ideological battle for legitimacy.
Islamist sources in Syria said the pamphlet, distributed widely online by supporters of the al Qaeda offshoot, was a step in the group's campaign to establish the caliphate and familiarise people with Baghdadi.
Entitled "Extend your hands and pledge loyalty to Baghdadi", the pamphlet appeared to be based largely on a longer statement written last year by IS religious figure Turki al-Binali arguing why Baghdadi deserved to command the loyalty of believers.
Addressing itself "to whoever fought and still fights in the path of God ... to commanders of groups," the pamphlet traced Baghdadi's purported lineage to the Prophet Mohammad, outlined his studies of Islamic law, and listed his battlefield achievements while asking supporters to unite against the group's enemies.
A coalition of the United States and Arab allies including Saudi Arabia, which follows the ultraconservative Wahhabi school of Sunni Islam, has been carrying out air strikes against IS in Syria.
"Isn't it time for you to unite with your brothers? To establish and elevate your state? The enemy has allied to fight you, so unite to fight him," said the pamphlet.
The U.S.-led campaign has pushed some members of al Qaeda's affiliate in Syria, the Nusra Front, to pressure their leaders to reconcile with IS after the two groups clashed earlier in the year.
Among other historical religious scholars, the pamphlet quoted Mohammad ibn Abd al-Wahhab, founder of Saudi Arabia's Wahhabi school, whose 18th century alliance with the Saudi ruling family continues to underpin the country's politics.
The kingdom's religious establishment has campaigned against militancy in recent weeks, describing both Islamic State and al Qaeda as Islam’s foremost enemy. In August, the country's Grand Mufti urged young people to ignore calls to jihad from people representing "deviant principles".
One Islamist fighter in Syria who is not an IS member said the group regarded issuing biographical detail about Baghdadi as a necessary step in establishing him as caliph.
"After this statement it is now official in their eyes that it is the duty of Muslims to pledge allegiance to him," the fighter said. "They are telling the world, 'Here is our Commander of the Faithful.'"
IS has its roots as al Qaeda's affiliate in Iraq, but split with the central organization headed by Ayman Zawahri this year after falling out over its expansion into Syria.
Leading figures affiliated with al Qaeda this week launched an initiative to establish a "truce" with IS and asked the group to respond by the Muslim holiday of Eid al-Adha, which starts on Saturday.
The rise of IS has appeared to capture the imaginations of at least some militants previously affiliated with al Qaeda. Algerian Islamist militants who pledged their loyalty to IS kidnapped and beheaded a French hostage last month and militants in the southern Philippines have threatened to kill two German hostages in a show of solidarity with IS.
"Extend your hands to pledge allegiance to Baghdadi," the IS pamphlet said. "It is a wonder that some people - who are not among you - are content to give allegiance to tyrants for years, but not to give allegiance to the commander of the faithful."
Additional reporting by Mariam Karouny in Beirut and Angus McDowall in Riyadh; editing by Anna Willard