Iraqi troops battled to dislodge an al-Qaeda splinter group from the city of Tikrit on Monday after its leader was declared caliph of a new Islamic state in lands seized this month across a swath of Iraq and Syria.

Alarming regional and world powers, the Islamic State in Iraq and Syria (ISIS) claimed universal authority when it dropped the local element in its name and said its leader Abu Bakr al-Baghdadi, as leader of the Islamic state, was now caliph of the Muslim world — a medieval title last widely recognized in the Ottoman sultan deposed 90 years ago after the First World War.

"He is the imam and caliph for Muslims everywhere," group spokesman Abu Muhammad al-Adnani said in an online statement on Sunday, using titles that carry religious and civil power.

The move, which follows a three-week drive for territory by ISIS militants and allies among Iraqi's Sunni Muslim minority, aims to erase international borders drawn by colonial powers and defy Baghdad's U.S.- and Iranian-backed, Shia-led government.

It also poses a direct challenge to the global leadership of al-Qaeda, which has disowned it, and to conservative Gulf Arab Sunni rulers who already view the group as a security threat.

'A threat to all countries'

The Iraqi government has appealed for international help and has accused Sunni neighbours, notably Saudi Arabia, of having fostered Islamist militancy in Syria and Iraq. Iraqi army spokesman Qassim Atta said declaring a caliphate could backfire by underlining Baghdadi's group posed a risk to other nations:

"This declaration is a message by Islamic State not only to Iraq or Syria but to the region and the world. The message is that Islamic State has become a threat to all countries," he said. "I believe all countries, once they read the declaration, will change their attitudes because it orders everybody to be loyal to it."

Fighters from the group overran the Iraqi city of Mosul on June 10 and have advanced toward Baghdad, prompting the dispatch of U.S. military advisers. In Syria, ISIS has captured territory in the north and east, along the desert frontier with Iraq.

Mideast Iraq

This photo by the official website of Iraq's Interior Ministry claims to show Abu Bakr al-Baghdadi, leader of the Islamic State of Iraq and Syria. (Iraqi Interior Ministry/Associated Press)

The government of Prime Minister Nouri al-Maliki, with the help of Shia sectarian militia, has managed to stop the militants from reaching the capital but security forces have been unable to take back cities they abandoned in the fighting.

The army attempted last week to take back the city of Tikrit but was unable to seize the city. Helicopters hit ISIS positions around the city overnight. On the southern outskirts, a battle raged into Monday, residents in the area said.

Tikrit was the home city of Saddam Hussein, whose overthrow by U.S. forces in 2003 ended a long history of domination by Sunnis over what is today a Shia majority in Iraq.

The fighting has started to draw in international support for Baghdad, two and a half years after U.S. troops pulled out.

Armed and trained by the United States, Iraq's armed forces crumbled in the face of the ISIS onslaught and have struggled to bring heavier weaponry to bear. Only two aircraft — turboprop Cessna Caravans normally used as short-range passenger and cargo carriers — are capable of firing the powerful Hellfire missile.

The U.S. is flying armed and unarmed aircraft in Iraq's airspace but says it has not engaged in fighting.

U.S. ups military presence

The United States is again ramping up its military presence in Iraq, sending around 300 additional troops into the country as well as a detachment of helicopters and drone aircraft, the Pentagon said on Monday.

Pentagon spokesman Rear Adm. John Kirby said about 200 forces arrived on Sunday in Iraq to reinforce security at the U.S. embassy and its support facilities and Baghdad International Airport. Another 100 personnel were also due to move to Baghdad to "provide security and logistics support."

"These forces are separate and apart from the up to 300 personnel the president authorized to establish two joint operations centres and conduct an assessment of how the U.S. can provide additional support to Iraq's security forces," Kirby said in a statement.

Russia, meanwhile, has sent its first warplanes to Baghdad, filling an order for five second-hand Sukhois. The government said they will be operational within a few days.

In Falluja, just west of Baghdad where ISIS fighters have been in control for six months, a bank accountant who asked to remain anonymous for fear of retribution said the announcement of the caliphate was a "step backward".

"It will only turn the government even more hostile to us," he said. "This will isolate us further from the rest of the world."

ISIS has used alliances with other, less radical Sunni armed groups and tribal fighters who are disillusioned with Maliki. Members of Saddam's secular Baath party have also fought in the revolt.

Caliphate declaration could cause in-fighting

Fawaz Gerges, a Middle East expert at the London School of Economics, said he expected the declaration would alienate ISIS's allies: "The strategic goal of the Baathists' is the capture of Baghdad, not the establishment of the caliphate.

"[ISIS's] pronouncement will most likely intensify the intra-jihadist struggle and widen the split between [ISIS] and its insurgent Sunni allies in Iraq," he said.

The term caliph indicates a successor to the prophet Mohammad, with temporal authority over all Muslims.

SYRIA-CRISIS/IRAQ/

A member loyal to the Islamic State in Iraq and Syria waves an ISIS flag in Raqqa, Syria, on Sunday. Muslim extremists have long dreamed of recreating the Islamic state, or caliphate, that ruled over the Middle East, much of North Africa and beyond in various forms over the course of Islam's 1,400-year history. (Reuters)

Traditionally it denotes a political and military leader with religious elements. Rival claims to the succession lie at the root of the 7th century schism between Sunnis and Shias.

Following Turkey's defeat in the First World War and the carving up of its Middle East empire by Britain and France, new Turkish nationalist rulers in 1924 formally abolished the caliphate that Ottoman sultans had held for nearly five centuries.

For many militant Islamists, who see a decline in religious observance and divisions among Muslims as causing many problems, the restoration of the caliphate has been an important goal.

According to the mid-20th century Egyptian Islamist writer Sayyid Qutb, whose ideas later helped form those of al-Qaeda, in order to bring about a new caliphate, at least one state must revive Islamic rule — a role al-Qaeda founder Osama bin Laden thought in the 1990s might be filled by Taliban-run Afghanistan.

Since the Ottoman collapse, Sunni Islam has lacked an internationally recognized clerical hierarchy. Senior figures generally hold authority within a single country. Among the most prominent of these is the Grand Mufti of Egypt, whose spokesman dismissed the new caliphate in Iraq and Syria as an "illusion".

"[ISIS's] announcement of what they called the Islamic caliphate is merely a response to the chaos which has happened in Iraq as a direct result of the inflammation of sectarian conflict in the entire region," Ibrahim Negm said in Cairo.

Extreme violence has alienated other Islamists

ISIS has followed al-Qaeda's hardline ideology, viewing Shias as heretics, but has alienated bin Laden's successor Ayman al-Zawahiri and other Islamists with its extreme violence.

Its declaration of the Islamic state could isolate allies in Iraq and lead to in-fighting. Such internal conflicts among rebel groups in Syria has killed around 7,000 people there this year and complicated the three-year uprising against Syrian President Bashar al-Assad, another ally of Shia Tehran.

IRAQ-SECURITY/

A Shia volunteer who joined the Iraqi army to fight against predominantly Sunni militants from the radical Islamic State of Iraq and Syria looks on during a parade in Kanaan. A drive for territory by ISIS militants and allies among Iraqi's Sunni Muslim minority aims to erase international borders and defy Baghdad's Shia-led government. (Reuters)

The group crucified eight rival rebel fighters in Syria, a monitoring group said on Sunday. And in the Syrian city of Raqqa, controlled by ISIS, militants held a parade to celebrate the declaration of the caliphate.

ISIS posted pictures online on Sunday of people waving black flags from cars and holding guns in the air, the SITE monitoring service said.

Some analysts say the group is a credible threat to frontiers and is stirring regional violence while others say it exaggerates its reach and support through sophisticated media campaigns.

The Islamic State also released a video called "Breaking of the Borders", promoting its destruction of a frontier crossing between the northern province of al-Hasakah in Syria and Nineveh province in Iraq, said SITE, which tracks militant websites.

With files from The Associated Press