Baghdad: US Secretary of State Hillary Rodham Clinton on Saturday assured Iraq that the Obama administration would not abandon the country even as it presses ahead with plans to withdraw American troops amid a recent surge in violence.

Clinton, on an unannounced trip to Baghdad, said the drawdown would be handled in a "responsible and careful way" and would not affect efforts to improve the professionalism of Iraq's security forces or reconstruction and development projects that are to be expanded.

But she stressed that Iraqis themselves - particularly within the security forces - would have to come together and overcome sectarian and other differences if they are to build a united, secure nation.

"Let me assure you and repeat what President Obama said, we are committed to Iraq, we want to see a stable, sovereign, self-reliant Iraq," she told a nervous but receptive crowd at a town hall meeting at the US Embassy here.

"We are very committed, but the nature of our commitment may look somewhat different because we are going to be withdrawing our combat troops over the next couple of years," Clinton said.

Making her first trip to Iraq as America's top diplomat, Clinton said the country has made great strides despite an explosion in violence that killed at least 159 people on Thursday and Friday.

Ahead of her arrival, Clinton said the attacks are a sign that extremists are afraid the Iraqi government is succeeding.

"I think that these suicide bombings ... are unfortunately, in a tragic way, a signal that the rejectionists fear that Iraq is going in the right direction," Clinton told reporters aboard her plane.

Clinton arrived a day after back-to-back suicide bombings killed 71 people outside the most important Shiite shrine in Baghdad. Those attacks came after Iraq on Thursday was rocked by its most deadly violence in more than a year when 88 people were killed by suicide bombers in Baghdad and Muqdadiyah, north of the capital.

Although overall violence is at its lowest levels since the months following the 2003 US-led invasion, the latest bombings come amid an increase in high-profile sectarian attacks that have raised concerns about the abilities of Iraq's security forces.

They have exposed gaps in security as Iraq takes over from US forces in protecting the country and Iraqi Prime Minister Nouri Al Maliki has ordered a military task force to investigate the attacks as well as shortcomings that allowed the assailants to slip through. The government on Saturday also ordered heightened security at major Shiite shrines across the country.

The Pentagon plans to hand over responsibility for most urban security in about three months as part of the administration's goal of a complete exit of forces by the end of 2011.

US officials say they are still committed to a June 30 deadline to move all forces outside major cities, including Baghdad. But the top US commander in Iraq, General Raymond Odierno, has said American troops could "maintain a presence" in some cities if requested by the Iraqis.

"Frankly, some people are afraid," said one participant in the town hall who said many questioned the ability, competence and neutrality of Iraq's security forces given the US withdrawal plan.

"There is nothing more important than to have a united Iraq," Clinton replied. "The more united Iraq is, the more you will trust your security services. The security services have to earn your trust but the people have to demand it."

"We will be working closely with the Iraqi government and the Iraqi security forces as we withdraw our combat troops, but we need to be sure that all of you are supporting a strong, non-sectarian security force and we will work to try and help make that happen," she said.

Clinton was met at the airport by the chairman of the US Joint Chiefs of Staff Admiral Mike Mullen, also on an unannounced trip to Baghdad, and the just-arrived new US Ambassador to Iraq Christopher Hill, who presented his credentials to the Iraqi government late Friday.

After a security briefing with Odierno, a meeting with Iraqi war widows and the town hall, Clinton was seeing Al Maliki, President Jalal Talibani and Foreign Minister Hoshyar Zebari.