Security forces killed at least 13 militants in overnight clashes in eastern Baghdad, the US military said Saturday. The army said Abrams tanks and drone-launched Hellfire missiles were used to quell attacks on US and Iraqi soldiers. However, Iraqi police and hospital officials said seven civilians died when US helicopters fired on homes and shops in Baghdad's eastern district of Sadr City early Saturday. They spoke on condition of anonymity because they are not allowed to release information. Sadr City is a principal stronghold of the Mahdi Army militia of cleric Muqtada al-Sadr. Government troops supported by the US military have been fighting to gain control of the district of 2.5 million people for nearly two weeks. On Friday, gunmen assassinated a top aide of al-Sadr, sharpening a Shi'ite power struggle that has already triggered fighting between the cleric's followers and the US-backed Iraqi government. Riyadh al-Nouri, director of al-Sadr's office in Najaf, was gunned down by an unknown number of assailants near his home after returning from prayer services, police and Sadrist officials said. Al-Sadr blamed the Americans and their Iraqi allies for the killing but called for calm - presumably to avoid a showdown at a time his Mahdi Army militia is under pressure by Iraqi and US-led forces in Baghdad and southern Iraq. Iraqi Prime Minister Nouri al-Maliki, also a Shi'ite, condemned "this savage crime" and ordered an investigation "to pursue and arrest the killers." But many of the 5,000 people who attended al-Nouri's funeral later Friday in Najaf chanted "al-Maliki is the enemy of God" as they shouted slogans against al-Sadr's Shi'ite political rivals. Authorities declared a curfew in Najaf, the world's premier Shi'ite theological center 100 miles south of Baghdad. Security forces took to the streets in several major cities across the Shi'ite south. A curfew was also imposed in Hillah, where government troops clashed with al-Sadr's militia last month. The assassination of such an influential Sadrist figure is likely to increase tension between al-Sadr's movement and the Shi'ite-led government. Several prominent Sadrists described al-Nouri as a voice of moderation within the movement, arguing against an armed confrontation with the Americans and al-Sadr's Shi'ite rivals. He had also opposed a decision by the Sadrists last year to withdraw from al-Maliki's government. Al-Nouri, 41, was one of al-Sadr's closest aides. Al-Nouri's sister is married to one of al-Sadr's brothers. As director of the Najaf office, al-Nouri was al-Sadr's representative in the world's most prestigious center of Shi'ite learning. Al-Nouri and another top al-Sadr lieutenant, Sheik Mustafa al-Yacoubi, were detained by American soldiers in May 2004 in the killing a year earlier of a moderate Shi'ite cleric, Sheik Abdul-Majid al-Khoei, in Najaf shortly after the US-led invasion of Iraq. An arrest warrant was issued for al-Sadr himself but was never served. The warrant and the closing by US authorities of al-Sadr's newspaper triggered massive uprisings that engulfed Shi'ite areas of central and southern Iraq. Al-Nouri and al-Yacoubi were freed in 2005 as part of an agreement to end the Sadrist rebellion, which claimed several thousand lives. Tension between al-Sadr and other Shi'ite parties exploded into violence last month when al-Maliki launched an ill-planned offensive against Shi'ite militias and gangs in Basra, Iraq's second largest city. The offensive faltered after al-Sadr's militia launched attacks throughout the south and in Baghdad, where militants showered the US-controlled Green Zone with rockets and mortars, killing four Americans. Clashes have continued in Baghdad and Basra, despite al-Sadr's order March 30 for his militiamen to stand down under a deal brokered in Iran. American and Iraqi officials insist the Basra crackdown was not aimed at al-Sadr's political movement but at criminals and Iranian-backed splinter groups. But Sadrists believed their Shi'ite rivals in government were trying to weaken their movement before provincial elections this fall. One of those rival parties, the Supreme Islamic Iraqi Council, controls the security services in Najaf. Sadrist officials said al-Nouri was slain about 300 yards from a security checkpoint but that it took police about 10 minutes to respond to the sounds of gunfire. In his statement Friday, al-Sadr, who is believed to be in the Iranian holy city of Qom, blamed the killing on "the hands of the occupiers and their stooges reaching out traitorously and aggressively against our dear martyr," a reference to the US and its Iraqi allies. "I call upon Sadrist followers to be patient," said al-Sadr, who is under enormous pressure from all Iraqi political parties to disband his militia, his most important instrument of power. "The occupiers will not rest in our land as long as I am alive," he said. "We demand the government open an investigation and punish the criminals. We call upon all political and religious groups to work toward ending the killing of clerics." President Jalal Talabani, a Kurd, described the killing as an attempt "to destabilize the country" and encourage "fighting among brothers in religion." Soon after the assassination, a rocket slammed into the Palestine Hotel in Baghdad, blowing a large hole in the wall and devastating an unoccupied room. Police said three people were killed, but hotel staff said only two employees received minor injuries. It appeared the rocket had been fired from a Shi'ite militia area at the Green Zone, located directly across the Tigris River from the hotel, but had strayed off-target. Meanwhile, US President George W. Bush, defending his decision to halt withdrawals of US troops after July, said Saturday that Iraqis are shouldering more responsibility for securing their future. The United States will stay on the offense, support the Iraqi security forces and move toward an oversight role, Bush said in his latest effort to garner support for the unpopular war. He used his Saturday radio address to promote his war policy, even though his approval rating hit a new low of 28 percent in an AP-Ipsos polling this week. The president on Thursday said he would heed the advice of his top commander in Iraq, Gen. David Petraeus. After the current drawdown of US troops ends in July, Petraeus wants 45 days to evaluate security - followed by an indefinite period to reassess US troop strength in Iraq, where flare-ups of extremist violence are threatening to undercut security gains. "I've told him he'll have time he needs to make his assessment," Bush said. That stance guarantees a heavy American military presence in Iraq for the rest of Bush's presidency as the war grinds through its sixth year. The current total of 160,000 troops is scheduled to shrink to about 140,000 by the end of July. Defense Secretary Robert Gates said Thursday that he no longer thinks, as he did last fall, that it was possible for troop levels to drop to 100,000 by year's end. Democrats have criticized the Iraqi government for not making faster political progress while US troops continue to fight and die. Democratic leaders also chide Bush for, in their view, failing to answer questions about exactly what conditions would allow troops to come home more quickly. Bush said, however, that the troop drawdowns under way show that the military buildup he ordered last year has worked to stem violence. "Serious and complex challenges remain in Iraq," he said. "Yet with the surge, a major strategic shift has occurred. Fifteen months ago, extremists were sowing sectarian violence. Today, many mainstream Sunni and Shia are actively confronting the extremists. "Fifteen months ago, al-Qaida was using bases in Iraq to kill our troops and terrorize Iraqis. Today, we have put al-Qaida on the defensive in Iraq, and now we are working to deliver a crippling blow. Fifteen months ago, Americans were worried about the prospect of failure in Iraq; today, thanks to the surge, we've revived the prospect of success in Iraq." Bush said US forces are continuing to transfer more security responsibilities to the Iraqi security forces. He said Iraq's economy was growing, and that Iraq was assuming responsibility for nearly all the funding of large-scale reconstruction projects. On the political front, he said Iraq is planning to hold elections that will provide a way for Iraqis to settle disputes through the political process instead of through violence.