U.S. and Iraqi forces launched an offensive against al Qaeda and "Iranian-supported" Shi'ite militants across Iraq on Monday in anticipation of an expected surge in violence.

U.S. commanders fear militants will step up attacks on U.S. soldiers or launch a "spectacular" attack on civilians to try to influence the debate over the war in Washington, where a keenly awaited progress report on the new U.S. military strategy in Iraq is due to be presented to Congress in September.

In Baghdad, leaders of Iraq's divided Kurdish, Shi'ite and Sunni Arab communities held a series of bilateral talks ahead of an expected summit this week.

The summit is aimed at healing the deep mistrust that has paralysed Prime Minister Nuri al-Maliki's national unity government and plunged it into its worst crisis.

"Everything will be on the table. It is like the days when we were forming the government, except that Maliki himself is not going to be replaced," said a Shi'ite official familiar with the talks, who spoke on condition of anonymity.

Those taking part in Monday's preparatory bilateral talks were Maliki, Deputy President Tareq al-Hashemi, a Sunni and member of the Accordance Front; President Jalal Talabani, a Kurd; Deputy President Adel Abdul-Mahdi, a member of the powerful Shi'ite Supreme Islamic Iraq Council; and Masoud Barzani, the leader of Iraq's largely autonomous Kurdish region.

The U.S. military described Operation Phantom Strike as "a powerful crackdown" jointly carried out by Iraqi troops.

"It consists of simultaneous operations throughout Iraq focused in pursuing AQI (al Qaeda in Iraq) terrorists and Iranian-supported extremist elements," it said in a statement.

The U.S. military says Iran has stepped up its support for Shi'ite militias, giving them more weapons and training, to hasten the departure of U.S. troops. Iran denies giving any aid.

The statement gave no details of the operation or how many of the 162,000 U.S. troops now in Iraq were involved.