A homicide car bomber tore through a busy market in the Shiite holy city of Kufa on Tuesday morning, killing at least 16 people and wounding 70 in an attack sure to further enflame tensions between Iraq's Sunni and Shiite populations.

In response, local authorities closed the entrances to Kufa and its sister holy city of Najaf and imposed a vehicle ban around the revered shrines and mosques in the two towns, said Ahmed Duaible, a local government spokesman.

The homicide attack came a day after Iraq's Sunni vice president threatened to leave the Shiite-dominated government unless key unspecified amendments to the constitution were made by May 15 — a move that would plunge Iraq into a political crisis.

Iraqi Vice President Tariq al-Hashemi made the threat to lead a Sunni walkout from the Cabinet and parliament in an interview on a cable news network. He also said he turned down an offer by President Bush to visit Washington until he can count more fully on U.S. help.

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Prime Minister Nouri al-Maliki, a Shiite, said al-Hashemi did not mention the threat in a meeting late Monday, and Ali Baban, the Sunni planning minister, said Tuesday the Sunni bloc had no plans to quit the government.

Al-Hashemi called his meeting with al-Maliki an effort to "melt the ice" and seemed to back away from the threat.

"I can say that we can, God willing, build an ambitious future based on a real partnership and joint understanding. And I think it is very important to go ahead with the political project," he told reporters.

Ali al-Dabbagh, al-Maliki's spokesman, said the parliamentary committee on amending the constitution was scheduled to present its recommendations on May 15 and should be given a chance to work. "There should be a dialogue, not threats. No political endeavor can succeed with threats," he said Tuesday.

The 550-pound car bomb at Kufa exploded about 10 a.m. in an area that also included a school and the mayor's office, police said. The 16 killed included women and children, said Salim Naima, spokesman of the Najaf health department.

"It was a huge explosion, its force threw me a few meters away from my wife," said Hussein Abid Matrod, a 38-year old taxi driver who was shopping with his wife and suffered shrapnel wounds to his back and legs. "I saw many people on the ground as smoke mixed with dust, and the smell of the gunpowder was everywhere."

Panicked people ran through the corridors searching for their relatives at the Furat al-Awsat hospital in nearby Najaf. Women in black abayas, traditional Islamic cloaks, pounded their chests and faces in grief.

"We are poor people looking for anything to secure our livelihood and we have nothing to do with politics. Why do they do this to us?" asked Firas Abdul-Karim, a 23-year-old day laborer who was wounded in the blast.

The revered Kufa mosque was about 400 yards from the blast. Millions of Shiite Muslim pilgrims visit the shrines at Kufa and Najaf, home to top Shiite cleric Grand Ayatollah Ali al-Sistani as well as radical anti-U.S. Shiite cleric Muqtada al-Sadr.

The predominantly Shiite southern areas have seen a spike in violence and unrest, blamed in part on militants who have fled a security crackdown in Baghdad.

On April 28, a homicide car bomber killed 68 people in a crowded commercial area near two of Iraq's most sacred Shiite shrines in Karbala, 45 miles northwest of Kufa. That attack came two weeks after a car bombing killed 47 people killed and wounded 224 wounded in the same area.

Hillah, about 60 miles south of Baghdad, also has been hit by some of the deadliest bombings this year, including a double homicide attack that killed 120 Shiite pilgrims and another one that killed 73 people in a market. Kufa itself was struck by a Dec. 30 at the fish market that killed 31 people.

Also Tuesday, a roadside bomb went off next to a passing mini bus in the Shiite area of Zafaraniyah on the southeastern outskirts of Bagdad, killing three passengers and injuring five others, police said.

At least 68 people were killed or found dead nationwide on Monday, police said, including the bullet-riddled bodies of 30 men found in Baghdad — the apparent victims of sectarian death squads.

All but two were found in west Baghdad, including 17 in the Amil neighborhood where Sunni politicians have complained of renewed attacks by Shiite militiamen, said a police official who spoke on condition of anonymity because he was not supposed to release those details.

A homicide bomber struck a market on the outskirts of the city of Ramadi, west of Baghdad, Monday afternoon, killing eight people, said police Col. Tariq Youssef. About 15 minutes later, a second car bomb struck a nearby checkpoint, killing five people, including two policemen, Youssef said.

The attacks occurred in areas controlled by the Anbar Salvation Council, an alliance of Sunni tribes formed last year to drive Al Qaeda from the area. Council officials blamed the attacks on Al Qaeda.

"They committed this crime because we have identified their hideouts and we are chasing them," said Sheik Jabbar Naif al-Dulaimi.

In a Web statement Monday, an Al Qaeda front organization, the Islamic State of Iraq, warned Sunnis against joining the government security forces — a move supported by the Salvation Council.

"We tell every father, mother, wife or brother who does not want to lose a relative to advise them not to approach the apostates and we swear to God that we will use every possible means to strike at the infidels and the renegades," the group said.

The Islamic State also claimed responsibility for attacks that killed 34 people over the weekend.

Also Monday, the military announced a U.S. soldier was killed by small-arms fire in western Baghdad the day before, bringing to nine the number of American personnel slain Sunday.

The security situation in the capital figured high in talks between al-Maliki and Bush, who spoke Monday in a video conference.

Al-Maliki told Bush of the need to maintain cooperation between U.S. and Iraqi forces as they continue their campaign to end the chaos and violence in Baghdad, the prime minister's office said in a statement.

White House spokesman Tony Snow said Bush and al-Maliki spoke about the Iraqi leader's push for political reconciliation, which is considered vital to bring stability.

"The prime minister is working with the presidency council to advance the political process in Iraq, including a lot of the legislation that we've been discussing over the last few months," Snow told reporters. "But issues of communications and reconciliation were at the fore."

Al-Maliki, a Shiite, reiterated his determination to work with Sunni leaders, Snow said.

But al-Maliki's government remains burdened by "narrow agendas" standing in the way of unity and crucial U.S.-backed legislation, such as a proposed law to share Iraq's oil wealth, said Gen. David Petraeus, the top U.S. commander in Iraq. Petraeus spoke Monday to the annual meeting of The Associated Press.

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