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 most deadly strikes on US forces in Baghdad since last summer's US troop surge.

It came hours after a Sunni tribal leader, Thaer Ghadban al-Karkhi, was killed in a suicide bomb attack at his house near Baquba, north of Baghdad.

Most vulnerable

Iraqi army spokesman Maj Gen Qasim Ata told AFP news agency: "A terrorist wearing an explosive vest blew himself against a dismounted US patrol.

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

"We remain resolute in our resolve to protect the people of Iraq and kill or capture those who would bring them harm," Col Allen Batschelet, chief of staff of US forces in Baghdad, told Reuters news agency.

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.

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

In the earlier attack, a woman detonated an explosives vest as Mr al-Kharki answered his door, killing him, his daughter and two guards.

He 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.