BAGHDAD — Iraqi forces have withdrawn from the militant-held city of Tikrit after their new offensive met heavy resistance, in a blow to the government effort to push back Sunni insurgents controlling large parts of the country.

The failure highlights the difficulties of Baghdad’s struggle to recapture territory from the insurgents who seized Mosul, Tikrit and other cities last month in a rapid offensive which threatens to fragment Iraq on ethnic and sectarian lines.

The setback came as Iraqi politicians named a moderate Sunni Islamist as speaker of parliament on Tuesday.

That was a long-delayed first step toward a power-sharing government urgently needed to confront the militants, who are led by the Al-Qaeda offshoot Islamic State.

It is unclear if the election of Salim Al-Jabouri as speaker will break the broader deadlock over Prime Minister Nouri Al-Maliki’s bid to serve a third term.

He has ruled since the April election as a caretaker.

Government troops and allied Shiite volunteer fighters retreated from Tikrit before sunset on Tuesday to a base four km (2.5 miles) south after coming under heavy mortar and sniper fire, a soldier who fought in the battle said.

Residents said there was no fighting on Wednesday in Tikrit, which lies 160 km north of Baghdad. It is a stronghold of ex-army officers and loyalists of executed former leader Saddam Hussein’s Baath Party who allied themselves with the militant offensive last month.

The stunning advance in the north and west by the militants over the past month has put Iraq’s very survival in jeopardy, as its politicians remain divided over forming a government to confront the insurgency.

A shared resentment at Maliki’s style of rule, which his critics say has marginalized Iraq’s Sunnis and Kurds, may have bolstered the Islamic State’s offensive last month.

The Shiite leader has defied demands from Sunnis and Kurds that he step aside for a less polarizing figure.

After quickly picking Salim Al-Jabouri as speaker on Tuesday, lawmakers argued bitterly for hours over his Shiite deputy, suggesting they are still far from a deal to complete the formation of a new government or a decision on the fate of Maliki. — Reuters