Bombs killed 21 people in central Baghdad on Wednesday, and gunmen attacked the governor's office in Iraq's second-largest city — another sign of unrest in the Shiite heartland as U.S. troops step up operations in the capital after the deadliest month for civilians of the war.

One bomb exploded late in the morning near day laborers waiting for work in the central Nahda district, killing eight people and wounding 28, police Lt. Bilal Ali said.

Two nearly simultaneous car bombs exploded Wednesday evening in the Batayween area of central Baghdad, killing 13 people and wounding 55, police Lt. Ali Mutaab said. The blasts sent a huge cloud of black smoke over the troubled city.

The attacks underscored the challenge facing U.S. and Iraqi officials as they seek to quell the surge in Sunni-Shiite violence which has been rising steadily since the Feb. 22 explosion at a Shiite shrine in Samarra, triggering a wave of reprisal killings.

Deputy Health Minister Adel Muhsin said about 3,500 Iraqis died last month in sectarian or political violence nationwide. He said it was the highest monthly death toll for civilians since the war started in March 2003.

Last week, the ministry said about 1,500 violent deaths were reported in the Baghdad area alone in July. U.S. commanders are rushing nearly 12,000 U.S. and Iraqi troops to the capital to try to end the carnage.

Violence also flared Wednesday in the southern city of Basra and in the northern city of Mosul.

In Basra, 340 miles southeast of Baghdad, gunmen from the Bani Assad tribe attacked the provincial governor's office because they believed officials were behind the killing of a tribal leader Tuesday, according to an official who was trapped for a time in the building.

The tribesmen fled after British armored vehicles arrived. Basra Gov. Mohammed al-Waeli said one policeman was killed and four were wounded. Seven attackers were arrested, he added.

Authorities imposed an indefinite curfew on the city to allow tempers to cool.

In Mosul, 225 miles northwest of the capital, armed clashes erupted between police and assailants in three neighborhoods on the mostly Sunni Arab western side of the Tigris River, police Lt. Col. Abdul-Karim Ahmed Khalaf said.

At least five gunmen were killed and six were arrested, he said. The clashes occurred one day after a suicide car bomber killed nine people in an attack on the Mosul headquarters of the Patriotic Union of Kurdistan, a Kurdish party headed by President Jalal Talabani.

The violence in Basra came one day after street battles in another southern Shiite city, Karbala, left 12 people dead, including two Iraqi soldiers. The trouble started after police raided the office of anti-American cleric Mahmoud al-Hassani.

Al-Hassani's followers took to the streets, firing Kalashnikov rifles, machine guns and rocket-propelled grenades at security patrols. The city was reported quiet but tense Wednesday.

Elsewhere in the Shiite area, a roadside bomb exploded near an Iraqi army patrol north of Hillah, killing three soldiers and wounding four, police 1st Lt. Osama Ahmed said. Hillah is a mostly Shiite city about 60 miles south of Baghdad.

A Danish soldier was shot in the back Wednesday in Qurnah, about 35 miles north of Basra, the Danish military said without giving further details.

An Iraqi militant group Wednesday released a video showing a Katyusha rocket purportedly fired at the U.S.-controlled Green Zone in a gesture of solidarity with Shiite guerrillas in Lebanon.

The footage obtained by The Associated Press showed several masked men casually setting up a launcher in a parking lot containing a number of burned-out buses before firing the rocket, which streaked across the sky out of view.

The group said the rocket was fired Sunday to demonstrate support for Hezbollah guerrillas who battled Israeli forces in Lebanon until a U.N. cease-fire ended 34 days of fighting Monday.

As they prepared to fire the weapon, the militants showed little concern that they might be discovered by a ground or aerial security patrol.

The tape featured a written statement declaring: "Our attacks against America, the sponsor of terrorism, will continue until the Zionist aggression against our people in Lebanon stops."

U.S. Ambassador Zalmay Khalilzad told The New York Times that Iran was encouraging Shiite militias to step up attacks on U.S. forces in retaliation for the Israeli assault on Hezbollah, which is backed by Iran.

Iran's prodding has led to a surge in mortar and rocket attacks on the fortified Green Zone that houses the main components of the Iraqi government and the U.S. Embassy, he said. Four Australian soldiers were wounded Monday in a rocket attack on the Green Zone.