Baghdad, Iraq ? A suicide car bomber flattened a restaurant in 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 – strongholds of radical Shiite cleric Muqtada al-Sadr’s Mahdi Army militia – 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 U.S. 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.