George Bush, the US president, has long resisted setting a timeline for withdrawal, but in July the White House began speaking of a general "time horizon" and "aspirational goals" to withdraw.

Iraq's leaders have become more confident of their ability to provide security as the country has become safer, but Zebari's comments came as fighting and bomb attacks across Iraq killed at least 11 people, including a US soldier.

Roadside bombings

The US soldier and four other people were killed in an attack involving a roadside bomb, a suicide bomber and small arms fire in Tarmiya, 25km north of Baghdad, the US military said.

It said two US soldiers, three Iraqi policemen and 18 other people were also wounded in the fighting.

In another attack, a suicide attacker blew up a bomb-laden minibus, killing at least three people and wounding 20 others in the town of Khanaqin, a security source said.

In Baghdad, two separate roadside bomb attacks killed at least six people, including one Iraqi soldier, and wounded at least 17 others.

Another roadside bombing targeting a private security company wounded at least four people.

Despite the violence, Iraq has taken an increasingly assertive stance in negotiations with the US after Nuri al-Maliki, the prime minister, ordered cracked downs on fighters earlier in the year.

But US military commanders say they worry that a hasty withdrawal could allow violence to resume.

Iraqi politics have also been paralysed by a dispute over the northern city of Kirkuk, which Kurds claim as the capital of their autonomous homeland.

The issues threatens to stoke ethnic tensions between the city's Kurds, Arabs and ethnic Turkmen.

The quarrel scuppered a law needed to allow provincial elections across the country, despite intensive lobbying by the US and UN to reach a deal.