A homicide bombing in the north and street battles hundreds of miles away in a Shiite holy city in the south claimed 16 lives Tuesday, demonstrating the tenuous security in Iraq as the U.S. focuses on curbing sectarian violence in Baghdad.

Nine people died in the suicide attack outside the regional party headquarters of Iraq's president in the northern city of Mosul; seven were killed in the fighting between Iraqi forces and followers of an anti-American cleric in Karbala.

In Baghdad, meanwhile, the U.S. military said car bombs triggered deadly explosions in a Shiite neighborhood Sunday, backing away from assertions that the blasts were caused by an accidental gas leak.

The suicide driver detonated his vehicle at the Mosul office of the Patriotic Union of Kurdistan party of President Jalal Talabani, killing five civilians and four security guards, police Col. Abdul-Kareem Ahmed al-Jibouri said. Forty-one people were wounded by the blast, which heavily damaged the one-story building and set 17 cars on fire, he said.

In Karbala, a Shiite holy city 270 miles south of Mosul, gunbattles broke out after police raided the office of Mahmoud al-Hassani, a Shiite cleric known for anti-American and anti-Iranian views. Police said they were searching for weapons.

CountryWatch: Iraq

Al-Hassani's followers responded by attacking police stations and checkpoints in at least five areas of the city, residents and officials said. Gunmen in civilian clothing fired Kalashnikov rifles, machine guns and rocket-propelled grenades at Iraqi army patrols.

A regional health official said three Iraqi policemen and four gunmen were killed and 17 people were wounded in the clashes. The official spoke on condition of anonymity because he is not authorized to speak to the media.

Officials ordered an indefinite curfew on the city, which contains some of the world's most sacred shrines for Shiite Muslims. But the curfew was widely ignored as gunmen roamed the streets, firing at police and soldiers.

"We have asked for extra forces from neighboring provinces to control the situation after clashes erupted between al-Hassani's supporters and the security forces," said Ghalib al-Daami, a member of Karbala's provincial council.

Late Tuesday, residents said tensions were rising in another Shiite city, Nasiriyah, with al-Hassani's followers brandishing weapons and blocking some streets.

Al-Hassani gained prominence for his nationalistic stand, calling for an Iraq free of influence from the Americans and Shiite-dominated Iran. Other key Shiite figures have sought to dampen his influence, which is mostly in Karbala and Basra, Iraq's second-largest city.

The U.S. command is rushing 12,000 U.S. and Iraqi reinforcements to curb unrest in the capital, which U.S. Ambassador Zalmay Khalilzad has described as the greatest threat to Iraq's future.

However, many other parts of the country remain unstable after three years of the U.S.-led international military presence, including the Sunni insurgent stronghold of Anbar and Basra, where British forces have failed to prevent Shiite militias from infiltrating the police and security services.

In Baghdad, U.S. and Iraqi forces have surrounded the Sunni neighborhoods of Amariyah and Ghazaliyah and the Shiite area of Shula, systematically searching houses and shops for weapons, residents said Tuesday.

One resident of Amariyah said the troops have allowed people to move about the streets but food and other goods were running low in the shops because the extra security had prevented merchants from restocking their shelves.

Those neighborhoods have been the scene of many killings, shootings and other violent incidents between Sunni and Shiite extremists.

Many Sunnis do not trust the Shiite-dominated police and army, and the presence of the Americans is aimed at reassuring them that the law will be enforced without regard to religious affiliation.

So far, however, the security operation has not resulted in a significant reduction in violence. On Sunday, 63 people were killed and about 140 were wounded in a series of explosions in Zafraniyah, a Shiite district of southeastern Baghdad.

Iraqi officials said the blasts were caused by car bombs and a rocket barrage from Dora, a mostly Sunni area where U.S. troops operate.

On Monday, U.S. military spokesman Maj. Gen. William B. Caldwell had said the destruction in Zafraniyah was caused by a gas line explosion and there was no evidence of car bombs or rockets.

However, a U.S. military statement said Tuesday the blasts were triggered by two car bombs that "detonated in the vicinity of a building, causing a gas explosion." The combination of the explosions "resulted in the collapse of a residential building."

It said two more car bombs went off in a one-mile radius within a short span of time. Four buildings and 20 shops were destroyed, it said.