BAGHDAD — Iraqi officials tightened security around churches in Baghdad and in two mostly Christian towns on Monday and braced for possible violence this weekend when huge crowds visit a holy Shiite shrine in the capital.

A series of church bombings targeting Iraq's Christian minority killed at least four people Sunday, including one that happened as worshippers were leaving Mass in eastern Baghdad. Iraq's Christians have often been targeted by Islamic extremists, and many have fled the country despite an overall drop in violence in the past two years.

Fearing car bombs, authorities imposed vehicle bans in the towns of Tilkaif and Hamdaniyah. Both are predominantly Christian towns near the northern city of Mosul where an Iraqi soldier was killed Monday when a bomb attached to his private vehicle exploded, police said.

Daldar Zebari, deputy head of the Ninevah provincial council, which includes Mosul, said the Christian-dominated areas of Mosul were a priority for the security services. But he added: "We will make our best efforts to keep security for the province and all its citizens of all ethnic and religious backgrounds without exception."

Ethnic and sectarian tension also is running high in Kirkuk, another city in northern Iraq where Adm. Mike Mullen, chairman of the U.S. Joint Chiefs of Staff, made an unannounced visit on Monday. Kurds want to make oil-rich Kirkuk the capital of their autonomous region in the north despite Arab opposition.

Violence is sharply down in the war that began with the U.S.-led invasion in 2003, but militants still carry out lethal attacks. The U.S. military completed a withdrawal of combat forces from Iraqi cities to outlying bases last month as part of a plan to let Iraq take the lead on ensuring its own security.

Iraqi military spokesman Maj. Gen. Qassim al-Moussawi told reporters in Baghdad that Iraq will not need help from multinational forces in securing an event at a holy Shiite shrine in northern Baghdad. Hundreds of thousands of Shiites are expected on Saturday in Kazimiyah, a predominantly Shiite suburb, to commemorate the death of a revered Shiite imam.

At the same commemoration in 2005, nearly 1,000 Shiite pilgrims, most of them women and children, died in a stampede during a religious procession on a bridge near the shrine. Thousands of the pilgrims, who panicked when they heard unfounded rumors of a suicide bomber, crushed one another or plunged 30 feet into the muddy Tigris River.

Al-Moussawi said the plan to secure the event is "100 percent an Iraqi one."

He lamented the church bombings, but blamed security at one church, saying that a man pretending that his car had broken down asked people at the church if he could park his vehicle near the church. "A few minutes after his departure, it went off," al-Moussawi said.

Also on Sunday, a bomb exploded near a convoy of American personnel that included U.S. Ambassador Christopher Hill, though no one was injured. U.S. embassy spokesman Susan Ziadeh said Monday that an investigation was under way into the bombing, which occurred as the convoy was traveling through Dhi Qar province in southern Iraq.