The U.S. military death toll hit a seven-month high of 50 on Wednesday - with more than half the losses in Baghdad as American forces wage growing street battles against Shiite fighters.
Iraqi civilian deaths also remained high following the Iraqi government crackdown on Shiite militia factions - accused by Prime Minister Nouri al-Maliki of using residents as human shields during close combat in the teeming Sadr City slum.
The clashes in Sadr City - a base for the powerful Mahdi Army militia - show little sign of easing as Iraqi and U.S. troops try to exert control over an area containing nearly half of the Baghdad's population.
In the deadliest skirmish Wednesday, suspected Shiite extremists first attacked with mortars and machine guns, then drove up a U.S. checkpoint and opened fire. The U.S. military said seven militants were killed. At least 10 other militiamen died in other clashes, the military said.
But the growing violence in Baghdad also has taken a toll on U.S. forces.
At least five soldiers have been killed in the city since Tuesday, bringing the monthly count to at least 50 - 27 in Baghdad - in the deadliest month since September when 65 U.S. troops died.
Since the U.S.-led invasion in 2003, at least 4,062 U.S. military personnel have died in Iraq, according to an Associated Press count.
The U.S. military reported early Thursday that a soldier had been killed by an explosion Wednesday near a patrol in Ninevah Province.
Around Iraq, at least 1,080 Iraqi civilians and security forces were killed nationwide this month, or an average of 36 a day, according to an AP tally. That's down from March's total of 1,269, or an average of 41 per day.
But nearly 40 percent of the April deaths - 413 - occurred in Baghdad as violence returned to the capital, according to the AP figures compiled from reports from Iraqi police, hospital officials and government offices.
Civilian deaths have steadily risen this year, and spiked sharply after al-Maliki launched the offensive on Shiite militias on March 25 in the southern city of Basra. Fighting soon flared in Sadr City, which has become the epicenter of the battles.
It's difficult to determine the civilian toll from the ongoing clashes in Sadr City.
An Interior Ministry official, who spoke on condition of anonymity because he wasn't authorized to release the information, said a total of 479 people have been killed in Sadr City since the clashes began in late March. But the official could not break down the number of militants, Iraqi security forces and civilians.
Tahseen al-Sheikhly, the spokesman for the civilian side of Baghdad security operations, said 925 people had died and 2,605 were wounded in Sadr City. But he gave no timeframe or details about how the figure was reached.
The U.S. military blamed the increase in deaths to an effort by both Shiite and Sunni militants to reverse recent security gains. The fighting intensified after al-Sadr last week threatened to unleash an "open war" against U.S.-led forces.
"We have said all along that this will be a tough fight and there will be periods where we see these extremists, these criminal groups and al-Qaida terrorists seek to reassert themselves," U.S. military spokesman Maj. Gen. Kevin Bergner told reporters in Baghdad.
"So, the sacrifice of our troopers, the sacrifice of Iraqi forces and Iraqi citizens reflects this challenge," Bergner added.
The Iraqi prime minister also showed no indications of backing down.
Al-Maliki vowed that "no one can stop" the drive to disarm Shiite and Sunni extremists, including the Mahdi Army - which he accused of using civilians as human shields and hiding in residential areas.
"We can't build a state along with militias," he said during a news conference. "We want to build a single national army."
Al-Maliki said gunmen had killed the nephew of police Maj. Gen. Abdul-Karim Khalaf, an Interior Ministry spokesman who has overseen operations in Basra, by hanging him from an electricity pole in Sadr City.
Local officials also claimed a school in Sadr City was hit by a U.S. airstrike. AP Television News footage showed a collapsed girls' school, with desks hanging from the slanting floors. The U.S. military did not specifically comment about the school.
In Washington, the White House confirmed that President Bush on Tuesday called Abdul-Aziz al-Hakim, head of the Supreme Islamic Iraqi Council, the most powerful Shiite party backing the Iraqi government and a chief rival of al-Sadr's movement.
Associated Press writer Qassim Abdul-Zahra in Baghdad and the AP News Research Center in New York contributed to this report.