WASHINGTON - President Bush declared on Friday that Iraq stands at a defining moment as it struggles to put down heavily armed Shiite militias. New flare-ups of extremist violence are threatening to undercut security gains and could sway Bush's decision about U.S. troop drawdowns.

In Baghdad, Shiite extremists lobbed rockets and mortars against the U.S.-protected Green Zone, which has come under steady barrages this week. The U.S. sent a Hellfire missile into a Shiite stronghold in the city. And in the south, fighting escalated in Basra where the mettle of Iraqi security forces is being sorely tested.

"Any government that presumes to represent the majority of people must confront criminal elements or people who think they can live outside the law," Bush said at the White House. "And that's what's taking place in Basra and in other parts of Iraq. I would say this is a defining moment in the history of a free Iraq."

It's also a key juncture for Bush in the five-year-old war that has claimed 4,000 American lives, worn U.S. forces thin and dominated his presidency.

Bush said Prime Minister Nouri al-Maliki's crackdown in Basra against Shiite militias vying for control of the oil-rich region is a positive milestone in the birth of a democratic nation. The Iraqi prime minister's decision to move against enemy elements in Basra shows "evenhanded justice" and the Iraqi government's willingness to go after both Sunni and Shiite insurgents and outlaws, he said.

Just as important is how the violence plays out. The ability of Iraqi security forces to control places like Basra will color the president's decision on whether to order more U.S. troop withdrawals beyond the five U.S. brigades already returning home by July _ something that's already looking unlikely.

The renewed violence, which has followed months of relative calm, threatened to unravel a fragile cease-fire with followers of radical Shiite cleric Muqtada al-Sadr. How much U.S. troops are drawn into the fight will be a telling sign of the Iraqi forces' ability to protect the nation.

Bush stressed that those Iraqi forces remained in the lead, yet U.S. forces stepped deeper into the fight.

U.S. pilots assisting Iraqi forces conducted airstrikes on Basra on Friday. American jets dropped bombs in the city, marking a sharp escalation in the fight against insurgents the Pentagon accuses of having links to Iran.

Bush said he did not know what triggered al-Maliki to act.

"This was his decision," Bush said during a news conference with visiting Australian Prime Minister Kevin Rudd, who is pulling 550 Australian troops out of Iraq.

Bush said of al-Maliki: "It was his military planning. It was his causing the troops to go from point A to point B. And it's exactly what a lot of folks here in America were wondering whether or not Iraq would even be able to do in the first place."

While he praised the Iraqi government's actions, he cautioned that the situation in Iraq remains "dangerous and fragile." He urged patience.

"They're fighting some pretty tough characters, people who kill innocent people to achieve objectives," he said. "And, yes, there's going to be violence, and that's sad. But this situation needed to be dealt with, and it's now being dealt with."

The Iraqi military campaign has triggered Shiite uprisings in other parts of the country, and, if the violence is not contained it could force U.S. officials to rethink further cutbacks in troop levels.

Early next month, Bush is expected to endorse a temporary halt in the withdrawal of troops from Iraq. The top U.S. commander in Iraq, Gen. David Petraeus, has warned against shrinking the American force so rapidly that the gains in security will be compromised.

The latest tally of casualties over the past several weeks shows more high spikes than in previous months _ including several days with 100 or more U.S. military, Iraqi military and Iraqi civilian deaths.

Officials say they are not yet certain whether the upturn in casualties represents a trend of increased violence. And it is not clear that Petraeus will be able to make such a conclusion before he goes before Congress April 8.