Attacks on US soldiers had dropped since last summer's troop surge

Three other troops and an Iraqi interpreter were also injured in the blast, an army statement said.

The attack is one of the deadliest against US forces in the Iraqi capital since last summer's US troop surge.

It came hours after a leader of a Sunni tribal alliance fighting al-Qaeda in Iraq was killed in a suicide bomb attack at his home in northern Iraq.

Thaer Ghadban al-Karkhi died when a woman detonated an explosives vest as he answered the door at his home in Baquba. His daughter and two guards also died.

Mr al-Karkhi was a member of the mainly Sunni Arab Awakening councils, allied with the US military against al-Qaeda in Iraq.

Sunni militias have been credited with helping to bring down the level of violence in Iraq in recent months.

Most vulnerable

The US military told the BBC that the attack on its troops took place in the Mansour district of the capital.

"Five soldiers paid the ultimate sacrifice on behalf of the Iraqi and American people," said Col Allen Batschelet, chief of staff of US forces in Baghdad.

"We remain resolute in our resolve to protect the people of Iraq and kill or capture those who would bring them harm."

The strike takes the number of US troops killed since the US-led invasion of Iraq in 2003 close to 4,000, says the BBC's Hugh Sykes in Baghdad.

RECENT US TROOP DEATHS 9 February: Four killed by roadside bomb north-west of Baghdad 28 January: Five killed by roadside bomb in Mosul 9 January: Six killed entering a booby-trapped house north-east of Baghdad

As the invasion's five-year anniversary approaches, many Baghdad neighbourhoods have witnessed security improvements, with the number of attacks on American soldiers significantly down from last year.

The bomber had targeted the US military at their most vulnerable, as military patrols around the Iraqi capital are often conducted in armoured vehicles, our correspondent says.

But the US troops cannot function unless they get out of their armoured vehicles and engage with the people, he adds.