A suicide bomber detonated an explosives-laden car at an entry control point to the western town of Ramadi on Tuesday, killing two U.S. Marines and wounding three others, the military said.
One civilian was killed and two dozen other residents were wounded in the blast. Ramadi is the capital of the former Sunni insurgent stronghold of Anbar province, and has been relatively peaceful since local tribal leaders joined forces with the U.S. military against al-Qaida in Iraq.
In Baghdad's embattled Sadr City district, an unmanned drone killed two gunmen, while another remote-controlled aircraft crashed south of the capital, the U.S. military said Tuesday.
U.S. Apache attack helicopters and armed Predator drones have been launching daily strikes against militiamen clashing with Iraqi and U.S. troops in Sadr City, a sprawling district of 2.5 million people.
There has been a sharp increase in violence in the country since Prime Minister Nouri al-Maliki launched an offensive against Shiite militias in the southern city of Basra nearly a month ago. The fighting quickly spread to Sadr City, one of the strongholds of the Mahdi Army militia of anti-American cleric Muqtada al-Sadr, to which U.S. and Iraqi forces have laid siege.
Militiamen also have responded by repeatedly shelling Baghdad's U.S.-protected Green Zone, which houses the U.S. Embassy and offices of the Iraqi government.
Last weekend, al-Sadr, who is believed to be in Iran, threatened to declare full-scale war on the U.S.-backed government if attacks on his followers continue. And on Monday, top Sadrists warned that open warfare was a "strong possibility" if the government did not ease the pressure on the Mahdi militia.
A military statement said that a drone spotted two gunmen with a rocket-propelled grenade launcher late Monday and engaged them with a Hellfire missile. Both men were killed.
In another firefight in Sadr City, U.S. troops were hit by a roadside bomb and then attacked with small-arms fire. Troops returned fire and killed three attackers, a statement said.
But near Iskandariyah, a town 30 miles south of Baghdad, a Shadow reconnaissance drone crashed early Tuesday, local police said. The U.S. military said it was investigating the cause of the crash.
Unlike the much larger Predator, the Shadow is an unarmed lightweight craft equipped with a camera capable of producing color video in daylight and thermal images at night, which it conveys back to controllers on the ground.
April has been a bad month for the drones, which are routinely used to monitor strife-torn areas of the country. Earlier in the month, two Predators crashed in different parts of Iraq and one was lost in Afghanistan. All are believed to have suffered mechanical failure, since insurgents in both countries lack even rudimentary anti-aircraft weapons.
In Basra, a senior aide to Iraq's top Shiite cleric Grand Ayatollah Ali al-Sistani died on Tuesday, a week after being seriously injured in an assassination attempt. Two other al-Sistani representatives were injured in separate ambushes.
The attacks came just days after a top aide of al-Sadr was killed in Najaf, suggesting the violence could be part of an internal Shiite power struggle.