BAGHDAD, march 26: Heavy fighting continued for a second day on Wednesday in two of Iraq’s largest cities, as Iraqi ground forces and helicopters mounted a huge operation to break the grip of the Shiite militias controlling Basra, and Iraqi forces clashed with militias in Baghdad. The fighting threatened to destabilize a long-term truce that had helped reduce the level of violence in the five-year-old Iraq war.

A spokesman for the Iraqi military said 40 people had been killed and 200 wounded in the two days of fighting in the southern city of Basra, according to The Associated Press. The spokesman, Co. Karim al-Zaidi, did not specify how many of those killed and sounded were militiamen, Iraqi soldiers or civilians caught up in the fighting.

Iraq’s prime minister, Nouri al-Maliki, on Wednesday gave gunmen in Basra a three-day deadline to surrender their weapons and renounce violence, The Associated Press reported. And three United States citizens working for the US government in Baghdad were seriously wounded Wednesday in a mortar attack in the Green Zone, the diplomatic and government compound, Reuters reported, citing a US embassy spokeswoman.

The battles, along with indications in recent weeks that militia and insurgent attacks had already been creeping up, raised fears across Iraq that Moktada al-Sadr, the renegade Shiite cleric, could pull out of a cease-fire he declared last summer. If his Mahdi Army militia does step up attacks, that could in turn slow American troop withdrawals.

There were also serious clashes in the southern cities of Kut and Hilla.

In Basra, US and British jets roared through the skies, providing air support for the Iraqi military. The scale and intensity of the clashes in Baghdad kept many residents home. Schools and shops were closed in many neighborhoods.