Gen. David Petraeus took charge of US forces in Iraq, becoming the third commander in the war and declaring the American task now was to help Iraqis "gain the time they need to save their country." Petraeus took command Saturday under a glistening crystal chandelier in a former Saddam Hussein palace at Camp Victory. The media-savvy, Princeton-educated Petraeus, 54, spoke bluntly of the task before him that coincides with President George W. Bush's decision to send an additional 21,500 US troops to clamp off violence in Baghdad and nearby regions. "We will have to share the burdens and move forward together. If we can do that and if we can help the people of Iraq, the prospects of success are good," he said. "Failing that, Iraq will be doomed to continued violence and civil strife." Meanwhile, the US military reported the deaths of three more American soldiers, killed in an explosion Friday in Diyala province northeast of Baghdad. US and Iraqi forces have battled Sunni insurgents and Shiite militias in Diyala for months. The deaths raised to 36 the number of Americans killed in Iraq so far this month. At least 3,120 service members have died since the Iraq war started in March 2003, according to an Associated Press count. At least 51 Iraqis were killed or found dead Saturday across the country. In one attack in central Baghdad, a car bomb exploded in a shopping district, killing six civilians and wounding 14. Petraeus, whose appointment was announced in early January, takes command of the roughly 135,000-strong US force in Iraq after two previous tours: what was seen as a highly successful stint as head of the 101st Airborne Division in Mosul, and a second tour in charge of training Iraqi forces. Petraeus, who was only confirmed by the Senate on Jan. 26, assumed four-star general status in conjunction with taking over the command. "The stakes are very high. The way ahead will be hard, and there undoubtedly will be many tough days. But as I recently told members of the US Senate, hard is not hopeless," said Petraeus, who took over for 58-year-old Gen. George Casey. Lt. Gen. Ricardo Sanchez was Casey's predecessor and his tenure was marred by the Abu Ghraib prisoner abuse scandal. He has since retired. The Sunni insurgency broke out during Sanchez's command, and the conflict in Iraq took on the many of the aspects of a civil war during Casey's tenure. Casey had focused on attempts to train Iraqi forces and on plans to turn over control of security to the Iraqi Army and national police. While Petraeus spoke of "barbaric enemies who brag of inhuman acts ... in the name of religion," outgoing commander Casey, who will become Army chief of staff, addressed the religious infighting that is tearing Iraq apart. "It's no secret that sectarian violence ... has changed the dynamics of what Iraqis must face here on the ground," he said. "Everything's not as I would have expected it to be or wanted it to be on my way out, but that's kind of the way things are." Casey said he was still too close to the situation to evaluate his tenure. "The main point people will debate is whether I relied too much on Iraqi forces to carry the security load and too little on coalition forces," he said. "But I'm certainly not ready to say that's a mistake. I'll let history judge that." An adviser to Prime Minister Nouri al-Maliki said the Iraqi government hoped Petraeus' command would help a new joint US-Iraqi security plan. "It reflects the change in US policy in Iraq. We have so much hope that the security plan will succeed and that he (Petraeus) will be part of that success," said the adviser, Bassam al-Husseini. Neither al-Maliki nor Iraqi President Jalal Talabani attended Saturday's ceremony, though Talabani and his two deputies met Petraeus on Friday and discussed the security plan, a government statement said. The effort to pacify Iraq, especially in Baghdad, also includes Sunni-Shiite reconciliation efforts. On Saturday, dozens of Shiites and a handful of Sunnis gathered for the reopening of a Sunni mosque in Baghdad's Shiite militia stronghold of Sadr City. But the Sunni Endowment, which is in charge of all Sunni mosques across Iraq, gave a lukewarm response to the initiative. "We cannot guarantee the return of displaced families to Sadr City ... so the mosque that has no worshippers cannot relay its message," the endowment said in a statement, adding it welcomed "any step to return things to the way they were before." Sadr City is the headquarters of the Mahdi Army, the militia of radical Shiite cleric Muqtada al-Sadr. The fighters are blamed for much of the sectarian killing that has targeted minority Sunnis in a year of revenge killings after al-Qaida in Iraq bombers destroyed an important Shiite shrine north of Baghdad. Militia members are keeping a low profile and trying to improve their image as US and Iraqi forces launch a security sweep that the Shiite-led government has promised would not spare Shiite militiamen or Sunni insurgents. The command swap is part of Bush's overhaul of Iraq policy, including a new focus on Iran's role. The administration has accused Tehran of funneling money and weapons to Shiite militiamen and Sunni insurgents in Iraq, and US forces have launched raids on Iranian targets in Iraq. But little evidence, such as documents or other items collected in the raids, has been made public. National security officials in Washington and Baghdad have worked for weeks on a presentation intended to provide evidence of the administration's claims. US officials in Baghdad scheduled a media briefing for Sunday that was expected to detail allegations of Iranian involvement.