By Raheem Salman and Michael Georgy
BAGHDAD (Reuters) - Iraq's most influential cleric, Grand Ayatollah Ali al-Sistani, threw his weight on Friday behind the new prime minister, calling for national unity to contain sectarian bloodshed and an offensive by Islamic State militants that threatens Baghdad.
Speaking after Nuri al-Maliki finally stepped down as prime minister under heavy pressure from allies at home and abroad, the spiritual leader of Iraq's Shi'ite majority said the handover to Maliki's party colleague Haider al-Abadi offered a rare opportunity to resolve political and security crises.
Iraq has been plunged into its worst violence since the peak of a sectarian civil war in 2006-2007, with Sunni fighters led by the Islamic State overrunning large parts of the west and north, forcing hundreds of thousands to flee for their lives and threatening the ethnic Kurds in their autonomous province.
Sistani told the country's feuding politicians to live up to their "historic responsibility" by cooperating with Abadi as he tries to form a new government and overcome divisions among the Shi'ite, Sunni and Kurdish communities that deepened as Maliki pursued what critics saw as a sectarian Shi'ite agenda.
Abadi himself, in comments online, urged his countrymen to unite and cautioned that the road ahead would be tough.
Sistani, a reclusive octogenarian whose authority few Iraqi politicians would dare openly challenge, also had pointed comments for the military, which offered serious resistance when the Islamic State staged its lightning offensive in June.
"We stress the necessity that the Iraqi flag is the banner they hoist over their troops and units, and avoid using any pictures or other symbols,” Sistani said, in a call for the armed forces to set aside sectarian differences. Maliki was blamed for blurring lines between the army and Shi'ite militias.
Maliki ended eight years in power that began under U.S. occupation and endorsed Abadi, a member of his Shi'ite Islamic Dawa party, in a televised speech late on Thursday during which he stood next to his successor, surrounded by other leaders.
Maliki's critics at home and abroad had accused him of marginalizing the Sunni Muslim minority, which dominated Iraq until a U.S.-led invasion deposed dictator Saddam Hussein in 2003. This, they said, had encouraged disaffected Sunnis to back the jihadist fighters who have ordered religious minorities to convert to their brand of Islam or die.
They have threatened to march on the capital.
The appointment of Abadi, who has a reputation as a less confrontational figure, had drawn widespread support within Iraq but also from the United States and regional Shi'ite power Iran - two countries which have been at odds for decades.
"The regional and international welcome is a rare positive opportunity ... to solve all (Iraq's) problems, especially political and security ones,” Sistani said in comments which were relayed by his spokesman after weekly Friday prayers in the Shi'ite holy city of Kerbala, south of Baghdad.
After its capture of the northern metropolis of Mosul in June, a swift push by the Islamic State to the borders of Iraqi Kurdistan alarmed Baghdad and last week drew the first U.S. air strikes on Iraq since the withdrawal of American troops in 2011.
European Union foreign ministers were holding an emergency meeting on Friday to discuss the region's response to major crises including the conflict in Iraq.
In London, the British government said it would consider "positively" any request for arms from the Kurds to help them battle the militants who have seized much of Iraq,
The United States has asked European countries to supply arms and ammunition to the Kurdish forces, U.S. and European officials have said.
Prime Minister David Cameron has so far said Britain's response would be limited to a humanitarian effort, but London has also been transporting to Kurdish forces military supplies, such as ammunition, being provided by other nations.
"If we were to receive a request then we would consider it positively," a spokeswoman for Cameron said.
Several European governments, including France, Germany, the Czech Republic and the Netherlands, have said they will send arms to the Kurds or are considering doing so.
Abadi is in the sensitive process of trying to form a new government in a country beset by daily bombings, abductions and executions. He must rein in Shi'ite militias accused of kidnapping and killing Sunnis and persuading the once dominant Sunni minority that they will have a bigger share of power.
(Additional reporting by Sarah Young in London; Writing by David Stamp; Editing by Alastair Macdonald)