President Bush says the upsurge in violence in southern Iraq and parts of Baghdad represents a "defining moment" in the history of a free Iraq.
Mr. Bush Friday said efforts by Iraqi forces to subdue Shi'ite militias are going to take a while, but are a necessary part of the development of a free society. He said Iraq's government must be willing to confront those criminal elements.
The president spoke at a news conference with the Australian prime minister as Iraq's parliament held an emergency session aimed at ending the violence in Basra and other southern Iraqi cities. That unrest has killed more than 100 people over the past few days.
Coalition forces dropped bombs in Basra Friday for the first time in support of Iraqi ground forces.
In Baghdad, the U.S.-protected "Green Zone" came under renewed attack, leaving two Iraqi guards dead outside Vice President Tareq al-Hashemi's office.
Latest reports from the scene indicate the vice president was not there at the time of Friday's attack. A thick cloud of smoke rose over the zone, which houses the U.S. Embassy and much of the Iraqi government.
Militants have been bombarding the Green Zone since Sunday. At least two Americans have been killed in the attacks, and the U.S. State Department has urged all U.S. embassy personnel to stay inside reinforced buildings.
Earlier, Iraqi Prime Minister Nouri al-Maliki extended a deadline - due to expire Friday - for Shi'ite gunmen fighting government forces to surrender their weapons. Mr. Maliki said militants will be rewarded financially if they hand over their weapons by April 8.
The militants are mostly followers of radical Shi'ite cleric Moqtada al-Sadr, who has called for a political solution to the crisis. Clashes between Sadr's Mahdi Army and Iraqi forces in southern Iraq and Baghdad are straining a seven-month cease-fire called by the cleric.
Some information for this report was provided by AFP, AP and Reuters.