BAGHDAD - A suicide 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.

U.S. marines and Iraqi soldiers on patrol the Shiite district of Sadr City in Baghdad, Iraq, Tuesday, May 8, 2007. U.S marines and and soldiers and Iraqi soldiers searched a neighborhood for weapons and explosives. [AP]

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 suicide 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 with CNN. He also said he turned down an offer by President Bush to visit Washington until he can count more fully on US help, CNN said on its Web site.

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-US 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.

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