BAGHDAD, (Reuters) – U.S. troops said they killed 18 insurgents in fierce clashes in the volatile western Iraqi city of Ramadi, while the military said it was checking reports that a helicopter had gone down north of Baghdad on Friday.

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 erupted on Thursday and continued on Friday morning.

Ramadi, 110 km (68 miles) west of Baghdad, is the capital of Anbar province, heartland of the Sunni insurgency and the most dangerous place in Iraq for U.S. troops.

The U.S. military said in a statement that U.S. forces were attacked in Ramadi on Thursday evening by small arms fire. 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 the 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 unrelenting violence shows how far Iraq is 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.

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 at least 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.