BAGHDAD, Iraq (CNN) -- A strict curfew was extended indefinitely in the Iraqi capital Sunday as the death toll mounted from clashes between government troops and Shiite Muslim militants.

A member of the Mehdi Army carries his weapon in the streets of Basra, Iraq, on Saturday.

Fighting sparked by a government-led push against "outlaw" militias in the southern city of Basra had left more than 280 people dead by Saturday, according to Iraqi authorities.

The unrest has stretched across southern Iraq's Shiite heartland up to Baghdad, where a ban on pedestrian and vehicle traffic was kept in place just hours before it was due to expire Sunday morning.

U.S. warplanes and British artillery struck targets in Basra on Saturday, a British spokesman said.

Another Basra airstrike killed 16 "criminal fighters," and a joint U.S.-Iraqi patrol killed 13 more fighters in southeastern Baghdad's Suwayrah district, U.S. commanders reported.

Prime Minister Nuri al-Maliki compared the "outlaws" to al Qaeda and vowed not to leave Basra, where he is personally leading the operation, "until security is restored."

"We will continue to stand up to these gangs in every inch of Iraq," he said.

"It is unfortunate that we used to use say these very words about al Qaeda, when all the while, there were people among us who are worse than al Qaeda." Watch al-Maliki lash out »

Al-Maliki has given the militants until April 8 surrender their arms to a guns-for-cash program that was scheduled to end at midnight Friday.

Supporters of Shiite cleric Muqtada al-Sadr, whose Mehdi Army militia has borne the brunt of the fighting, say they have been unfairly singled out by the crackdown.

Al-Sadr has told followers not to surrender their weapons "except to a state that can throw out the occupation," a top aide, Salah al-Obaidi, said Saturday.

The violence has sparked fears that a seven-month cease-fire by the Mehdi Army -- regarded as a key factor in a dramatic drop in attacks in recent months -- could collapse or that the U.S. military will have to bail out the Iraqis.

Al-Sadr's political party holds 30 seats in Iraq's parliament and once held seats in al-Maliki's cabinet, quitting last year after the prime minister refused to set a deadline for U.S. and coalition troops to leave.

Basra, Iraq's second-largest city and its chief oil port, has been plagued by turf wars among al-Sadr's followers, the smaller Fadhila party and the country's largest Shiite party, the Islamic Supreme Council of Iraq, the major partner in al-Maliki's ruling coalition.

The prime minister met Saturday in Basra with the area's leaders, who have expressed support for the government's efforts to "impose law and save Basra from criminal gangs," according to a written statement from the prime minister's office.

Security forces went to Basra to fight "murder and smuggling gangs and outlaws," the prime minister said, and hadn't intended to fight certain groups -- apparently referring to the Mehdi Army.

Al-Sadr's militia launched two uprisings against U.S. troops in 2004. But in August, after a series of clashes between Mehdi Army fighters and security forces linked to the Islamic Supreme Council's Badr Brigades, he ordered his militia to suspend operations.

In an interview that aired Saturday on the Arabic-language news network Al-Jazeera, recorded before the current fighting broke out, al-Sadr compared al-Maliki to executed former dictator Saddam Hussein.

"Under Saddam's rule, we complained about how the government distanced itself from the people and operated under dictatorial terms. Now, the government is also dealing with people on such terms," al-Sadr said.

This week, President Bush called the current clashes a "defining moment" for Iraq and a key test for the country's government.

But several U.S. officials said Friday that the Iraqi military push is not going as well as American officials had hoped. A U.S. military intelligence analysis found that Iraqi security forces control less than a quarter of Basra, officials in both the United States and Iraq said.

"This is going to go on for a while," one U.S. military official said.

American troops have been supporting Iraqi forces with intelligence, surveillance and occasional airstrikes and raids in Baghdad, according to the U.S. military.

U.S. trainers have also accompanied Iraqi units into combat, as in Saturday's firefight in Suwayrah.

The U.S. military dropped two bombs Saturday afternoon at a suspected Shiite militia stronghold in the Basra area, said Maj. Tom Holloway, a British military spokesman.

The strikes were followed by shelling from the British garrison at the city's airport, aimed at mortar positions manned by militia fighters, he said.

Both attacks were in response to requests by Iraqi forces for air support, Holloway said. He added that coalition forces were investigating reports of civilian casualties but had no details.

Meanwhile, at least 40 members of Iraq's national police turned in their uniforms and joined forces with al-Sadr's militia in Baghdad, al-Obaidi said. They took their U.S.-supplied weapons with them, an Iraqi Interior Ministry official said.

Mortar and rocket attacks were directed Saturday at Baghdad's fortified International Zone, also known as the Green Zone, where Iraqi government buildings and embassies are located. No injuries were reported, a U.S. Embassy spokesman said. Watch the mayhem in Baghdad »

Other developments

• Two U.S. soldiers were killed Saturday by a roadside bomb in eastern Baghdad, and one was killed the same way Friday south of the city, the military said. The U.S. military death toll in Iraq now stands at 4,007.

• Turkey's military said it killed at least 15 rebels in operations in northern Iraq this week, but a spokesman for the Iraqi Kurdish Regional Security Forces denied the report, saying Turkey has not conducted any military operation or air assault there in the past two weeks. E-mail to a friend

CNN's Thomas Evans and Mohammed Tawfeeq contributed to this report.

All About Basra • Muqtada al-Sadr • Nuri al-Maliki • Iraq War