Iraqi officials say a bomb struck an oil pipeline Thursday in the southern city of Basra, where Iraqi forces are battling Shi'ite militias for a third day.

Officials did not say who was responsible for the pipeline attack. It is also unclear whether the damaged pipeline will greatly affect crude exports.

Meanwhile, witnesses in Basra reported more heavy fighting in the city today. Officials say at least 40 people have been killed since Iraqi forces began an operation against Shi'ite militias on Tuesday.

Iraqi Prime Minister Nouri al-Maliki has issued an ultimatum, ordering Shi'ite gunmen to surrender their weapons by Friday or face the consequences.

In Baghdad, followers of radical Shi'ite cleric Moqtada al-Sadr held demonstrations today against Mr. Maliki.

Separately, rockets or mortar shells struck Baghdad's heavily fortified Green Zone, a day after a similar attack wounded three Americans.

Officials also said renewed fighting in Baghdad's Sadr City district killed at least 14 people Wednesday and wounded more than 100 others.

White House spokesman Tony Fratto said the Bush administration applauds Mr. Maliki's decision to take action in Basra, saying the Iraqi government is trying to take control of its own security. He said the White House suspects Iran is assisting some insurgents in the region.

The clashes in Basra are jeopardizing a seven-month-old cease-fire Sadr declared last year.

Some information for this report was provided by AFP and Reuters.