Iraqi government forces backed by helicopter gunships began an offensive on Saturday to retake the northern city of Tikrit from Sunni Islamist militants while party leaders pursued talks that could end Prime Minister Nouri al-Maliki's divisive rule.

However, the militants were showing resilience and enjoyed the backing of some local Sunni tribes, as well as former ruling Baathists from the era of late Sunni dictator Saddam Hussein -- whose hometown was Tikrit -- alienated from Maliki's government.

In a stunning political intervention on Friday that mean the demise of Maliki's eight-year tenure, powerful Shi'ite cleric Grand Ayatollah Ali Sistani urged political blocs to agree on the next premier, parliament speaker and president before a newly elected legislature meets in Baghdad on Tuesday.

Under Iraq's governing system put in place after Saddam's overthrow, the prime minister has always been a Shi'ite, the largely ceremonial president a Kurd and the speaker of parliament a Sunni.

Allies of Maliki said Sistani's call for a quick decision was not aimed at sidelining the premier but at putting pressure on all political parties not to drag out the process with typical infighting with Iraq facing disintegration. Even so, they acknowledged Sistani was not happy with Maliki's policies.

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