(Adds attack on pilgrims) By Ibon Villelabeitia BAGHDAD, March 6 (Reuters) - Six U.S. soldiers were killed north of Baghdad on Monday in one of the worst ground attacks in months on U.S. forces in Iraq and three more died in a separate incident also outside the capital, the military said on Tuesday. The violence comes as American and Iraqi troops are stepping up a three-week old security crackdown in Baghdad aimed at stemming sectarian bloodshed. Commanders have said insurgents may intensify assaults outside the capital, where more than 90,000 Iraqi and U.S. troops have been deployed for the push. In the deadliest of the two attacks on U.S. forces, six soldiers were killed and three others wounded by a roadside blast near their vehicles in Salahaddin province, a Sunni Arab insurgent stronghold north of Baghdad, the military said. In the separate incident also on Monday, three U.S. soldiers were killed and one wounded by a blast near their vehicles in Diyala province, northeast of Baghdad. U.S. commanders are concerned about the increased use by insurgents of a particularly deadly type of roadside bomb which, U.S. commanders say, is made in Iran. The devices have killed more than 170 U.S. soldiers in Iraq since 2004. More than 3,170 U.S. soldiers have died in Iraq since the U.S.-led invasion in March 2003. In the latest of a string of attacks against Shi'ite pilgrims streaming to the holy city of Kerbala to mark an important religious event, five pilgrims were killed and 10 wounded by a car bomb in central Baghdad, police said. At least seven pilgrims were killed in several attacks in Baghdad on Monday. Sunni Arab insurgents frequently attack Shi'ite pilgrims and sites in what U.S. and Iraqi officials say is a campaign to spark a sectarian civil war. A day after a suicide bomber devastated Baghdad's historic booksellers' district, killing 30 people, residents pulled nine charred bodies from the smouldering rubble in Mutanabi Street, witnesses said. "I saw nine bodies being pulled out. They were completely burned. The firefighters could not reach them yesterday because the stores were full of books and papers and they were burning," said the witness, who works for Reuters. The attack was another challenge to Prime Minister Nuri al-Maliki, who has said he was pleased with the early results of the crackdown in Baghdad. For a second straight day on Monday, U.S. and Iraqi troops conducted door-to-door searches in Sadr City, a stronghold of the Mehdi Army Shi'ite militia. The Mehdi Army is commanded by anti-American Shi'ite cleric Moqtada al-Sadr, and the operations in Sadr City could test Iraqi and U.S. determination to enforce the crackdown. Washington has called the Mehdi Army the greatest threat to security in Iraq, but its leaders are lying low, unlike in 2004 when the militia twice rose up against American forces. (Additional reporting by Mussab Al-Khairalla and Aseel Kami)
