Shi'ite cleric Muqtada al-Sadr raised the stakes Tuesday in his showdown with the Iraqi government, threatening to end formally a seven-month cease-fire unless authorities stop attacks on his followers in Baghdad. A formal end of the cease-fire could trigger renewed fighting throughout southern Iraq, nine days after a deal brokered in Iran calmed the region. But there was no letup in the clashes in the capital Tuesday, as American and Iraqi soldiers stepped up the pressure against Shi'ite militants in their Sadr City stronghold of northeast Baghdad. US troops fired missiles at three mortar positions, killing 12 militants, the American command said. Iraqi police and hospitals said 14 people were killed and 37 wounded in Sadr City. Two more US troops were killed in the Baghdad fighting, the US command announced. At least 12 American service members have died in Iraq since Sunday. Also Tuesday, rockets or mortar shells also slammed into the US-protected Green Zone, but the US Embassy said there were no casualties. The bloodshed served as stark reminders of Iraq's continuing instability, five years after US troops swept into Baghdad and toppled Saddam Hussein's regime on April 9, 2003. The euphoria of victory was soon dissipated - first by a Sunni insurgency, then Sunni-Shi'ite slaughter and now battles against Shi'ite militiamen. In Washington, top US commander Gen. David Petraeus called Tuesday for an open-ended suspension of US troop withdrawals this summer because of concern over the renewed fighting. As tension rose in Baghdad on the eve of the anniversary, the Iraqi military ordered vehicles and motorcycles off the streets from 5 a.m. Wednesday until midnight - a move apparently aimed at preventing Shi'ite gunmen from moving freely about the city. The vehicle ban was imposed despite a decision by al-Sadr to call off his "million-strong" demonstration set for Wednesday to demand an end to the American military presence. Al-Sadr's Mahdi militia has been battling American and Iraqi soldiers in the sprawling Sadr City slum. Fearing the demonstration might trigger violence throughout Baghdad, Iraqi soldiers began turning back military-aged men traveling to the capital Tuesday from Shi'ite areas to the south. Al-Sadr then called off the rally, apparently fearing a modest turnout would display weakness at a time when he is locked in a violent power struggle with Prime Minister Nouri al-Maliki, a fellow Shi'ite. Al-Maliki has told al-Sadr to disband his militia or give up politics. Instead, al-Sadr's aides called a news conference at a hotel on Firdous Square, where US Marines hauled down the statue of Saddam five years ago. The aides released a statement condemning the government for allegedly bowing to "the hated American pressure." "I call on the Iraqi government, if it exists, to work to protect the Iraqi people, stop the spilling of its blood, and the abuse of its honor," al Sadr said in the statement. He also urged the government to "demand the withdrawal of the occupier or a schedule for its withdrawal from our holy land." Otherwise, al-Sadr said he might formally end the cease-fire he imposed on his Mahdi militia last August - a move that US officials acknowledge played a major role in calming the violence until last month. New fighting flared after al-Maliki led a major crackdown March 25 on Shi'ite militias and criminal gangs in the southern city of Basra. That triggered a violent backlash by Shi'ite militiamen, who launched attacks across southern Iraq and Baghdad including missiles against the Green Zone. Fighting eased after al-Sadr called on his militia March 30 to halt attacks. But clashes have continued in Sadr City, where US and Iraqi troops are trying to push militants out of rocket range of the Green Zone. The flare-up followed months of declining attacks, especially in Sunni areas of central and western Iraq. Thousands of Sunni tribesmen broke with al-Qaida in Iraq and joined with US and Iraqi troops to fight the terror movement. But the explosion of violence in the south underlined warnings by the US military that the security gains were reversible. It also illustrated the complexity of Iraq, where numerous religious sects and ethnic groups still compete for power. Iraqi military spokesman Brig. Gen. Qassim al-Moussawi said a total of 82 militants, 36 civilians and 37 soldiers had been killed since March 16 in fighting in Baghdad, mostly in Sadr City. Al-Moussawi also announced that gunmen in Baghdad's northern neighborhood of Kazimiyah - the site of Baghdad's most prominent Shi'ite religious shrine - have three days to hand in their weapons. But police in several cities in southern Iraq say few Shi'ite militants have given up their guns and that most are awaiting orders from al-Sadr to resume attacks. In Basra, Interior Ministry spokesman Maj. Gen. Abdul-Karim Khalaf confirmed that 1,000 Iraqi security troopers had "failed to carry out their duties" during last month's crackdown and would face disciplinary action. The failure of Iraqi troops to crush the militias has raised fresh doubts about the ability of the Iraqis to take over their own security so US-led forces can go home. In other violence Tuesday, Iraqi police said a roadside bomb struck a minibus near Balad Ruz, northeast of Baghdad, killing at least six morning commuters and wounding 10 others. Gunmen also attacked the home of a US-allied Sunni tribal leader near Baqouba, 50 kilometers north of Baghdad, killing him and three of his sons, police said.