(Adds U.S. attack on al Qaeda-linked cell) By Dean Yates BAGHDAD, Feb 2 (Reuters) - U.S. troops said they killed 18 insurgents in clashes in the volatile Iraqi city of Ramadi, and launched an air strike on Friday against an al Qaeda-linked insurgent cell in Baghdad responsible for suicide car bombings. The U.S. attack in southern Baghdad targeted a network responsible for a "large and devastating number" of suicide car bombings, the military said in a statement. "Coalition forces believe that key terrorist leaders were killed during the air strike," the statement said, adding the cell has also carried out roadside bombings and sniper attacks. The U.S. military said it was checking reports that a helicopter had gone down north of Baghdad. Three helicopters have come down in the past two weeks in Iraq, possibly all shot down. "We're looking at reports of a possible aircraft down," a U.S. military spokeswoman, Lieutenant Colonel Josslyn Aberle, said, when asked about reports from Iraqi police and residents near Taji that they saw a helicopter come down shortly after dawn. Taji is the site of a major U.S. air base. The fighting in Ramadi, 110 km (68 miles) west of Baghdad, erupted on Thursday and was still raging on Friday morning, underscoring unrelenting violence as Washington continues to debate the state of its military policy in Iraq. Ramadi, capital of Anbar province, is the heartland of the Sunni insurgency and the most dangerous place in Iraq for U.S. troops. The U.S. military said its forces came under small arms fire there on Thursday evening. When U.S. machinegun and tank fire failed to quell the attack, troops called in a missile strike. At least 15 insurgents were killed. U.S. forces were again attacked on Friday by militants armed with rocket-propelled grenades in several buildings. When clashes continued, troops again called in a missile strike. The military said three militants were killed. It said no U.S. troops were killed or wounded. The outgoing U.S. commander in Iraq, General George Casey, said on Thursday he had laid the foundations for success but the relentless tide of violence shows Iraq is far from being stabilised. Two suicide bombers killed 61 people and wounded 150 when they blew themselves up at a crowded market in the Shi'ite town of Hilla on Thursday, police said. SCATHING CRITICISM In Washington, Casey defended his record amid scathing criticism in Congress. "I do not believe that the current policy has failed," Casey told the U.S. Senate's Armed Services Committee, meeting to consider his nomination to be U.S. Army chief of staff. Even President George W. Bush referred to the Iraq policy under Casey as "maybe a slow failure" last month as he made his case for sending 21,500 more troops to Iraq in an effort to halt the slide to all-out sectarian war. Tens of thousands of Iraqis and more than 3,000 U.S. troops have died in Iraq since U.S.-led forces invaded in 2003 and toppled Saddam Hussein. Several senators asked Casey if he truly supported Bush's plan for more troops, as the general requested two extra combat brigades for Baghdad, not the five the president wants. Casey said the three further brigades would allow more flexibility and be deployed over time so they could be stopped if they were not required. That suggested a difference in emphasis from his designated successor, General David Petraeus, who has asked for all the extra forces to be deployed as quickly as possible. Shi'ite Prime Minister Nuri al-Maliki has vowed to tackle militants on both sides of the sectarian divide. (Additional reporting by Ross Colvin and Alastair Macdonald in Baghdad, and Andrew Gray in Washington0
