A woman wearing a vest lined with explosives blew herself up near Shiite worshippers in turbulent Diyala province north of the capital Wednesday, killing nine of them -- the latest in a growing number of female homicide attacks.

Six people were wounded in the bombing in Khan Bani Saad, a town 15 kilometers (nine miles) south of Baqouba, Diyala's provincial capital, police and hospital officials said.

According to residents and police, the woman detonated her explosives when she saw Shiite men in black making preparations about 50 yards from a mosque for a ceremony marking Ashoura, the holiest day in the Shiite calendar.

Sunni Arab militants have repeatedly targeted Ashoura processions since 2003, with hundreds killed by mortar shelling or car bombings. As a precaution, authorities announced a 48-hour ban on the use of vehicles in Baghdad and nine provinces south of the capital starting Thursday at dusk.

Ashoura, which comes later this week, commemorates the death in a 7th century battle of Imam Hussein, one of Shiite Islam's most revered saints whose tomb is in the city of Karbala, about 100 kilometers (60 miles) south of Baghdad.

Although female homicide bombings have been fairly rare in Iraq, extremists have been using women more frequently in recent months. U.S. officials say this indicates the militants are running short of male volunteers. However, it could also be that Al Qaeda in Iraq believes women are less likely than men to be searched and that explosives are easier to conceal under women's clothing.

Wednesday's bombing was the fourth female homicide attack in Iraq in three months. All have taken place in Diyala, which has been a major focus of a nationwide campaign the U.S. military launched last week against Al Qaeda in Iraq and other Sunni extremists.

Northwest of Diyala, meanwhile, small arms fire killed three U.S. soldiers conducting operations Wednesday in Salahuddin province, the military said. Two other soldiers were wounded and evacuated to a coalition hospital.

Diyala has defied the trend toward lower violence over the past six months in Baghdad and much of central Iraq. Insurgents who were pushed out of the western province of Anbar and out of Baghdad have shifted their operations into the farming region of palm and citrus groves, where Shiite and Sunni communities press up next to each other like squares on a checkerboard.

At least 273 civilians were slain in Diyala last month, compared to at least 213 in June, according to an Associated Press count. Over the same span, monthly civilian deaths in Baghdad dropped from at least 838 to at least 182.

But after several months of relative quiet in Baghdad, fighters believed allied with Iran have resumed mortar and rocket attacks, with several big blasts heard shortly after dawn on Wednesday as well as a few more later in the morning.

On Tuesday night, at least five mortars crashed into the fortified Green Zone, site of the American Embassy and Iraqi government, not long after U.S. Secretary of State Condoleezza Rice held a news conference after making an unannounced visit.

Mortar and rocket attacks on the Green Zone, which had been a daily event, virtually stopped about mid-October. The quiet followed a six-month cease-fire announced by radical Shiite leader Muqtada al-Sadr and his Mahdi Army militia in August, though some breakaway factions of al-Sadr's group continued to launch attacks.

The resumption of the attacks coincided with a sharp rise in U.S. rhetoric against Iran by U.S. President George W. Bush during his tour of the Middle East.

Two Mahdi Army commanders have told The Associated Press the uptick in mortar and rocket attacks is not the work of their organization, which continues its cease-fire.

Instead, they said the attacks are the work of a new organization with ties to Iran. The group, called Italat, which means "information" or "intelligence" in Farsi, was formerly the Iranian Revolutionary Guard's liaison to the Mahdi Army and its rogue factions, the commanders said. They spoke on condition of anonymity because they did not want to advertise their jobs to the U.S. military.

Not all the attacks in Baghdad may be linked to Shiite extremists. About 10 a.m., two mortar rounds slammed into Palestine Street in east Baghdad.

Three pedestrians were wounded, police said. The target was unclear, but the neighborhood is dominated by Shiites.

But other types of attacks linked to Iran also appear to be on the rise.

On Sunday, Gen. David Petraeus, the top U.S. commander in Iraq, told reporters that the overall flow of weaponry from Iran into Iraq appears to be down, but that attacks with "explosively formed projectiles" tied to Tehran are up by a factor of two or three in recent days. "Frankly, we are trying to determine why that might be," he said.

The roadside bombs, known as EFPs, are armor-piercing explosives that have killed hundreds of U.S. soldiers in Iraq. U.S. military officials have said for months that mainly Shiite Iran has been supplying the devices to Shiite militias in Iraq. Tehran denies it.

In other attacks Wednesday, a roadside bomb exploded at 8 a.m. in the commercial Bab al-Muadham district of Baghdad, killing two civilians and wounding four. The blast appeared to target a passing police car but instead hit a civilian car, a police officer said.

About the same time, another roadside bomb went off southeast of Baghdad at an intersection where U.S. and Iraqi troops often pass, police said. The attack killed one civilian and wounded four others.

All police spoke on condition of anonymity because they were not authorized to release information.