The US defence secretary has arrived in Baghdad on an unannounced visit intended to show US support for Iraq's new leadership.

Donald Rumsfeld's visit to the capital, aboard a military cargo plane, was his first to Iraq in 2006 and comes just days after Shia politician Jawad al-Maliki was chosen as prime minister

"The reason we're going into Iraq is because this is a very significant time and ... the president (George Bush) asked us to go in to show support for this new government," Pentagon press secretary Eric Ruff told reporters travelling with Rumsfeld.

The United States has urged al-Maliki, nominated as prime minister on Friday to get a new cabinet in place as soon as possible.

Elections were held in December, but Iraqi leaders agreed only last week to form a new coalition government after four months of political wrangling amid worsening sectarian violence and armed conflicts.

Rumsfeld, whose last visit to Iraq was in December, was due to meet Iraqi officials as well as being briefed by US Army General George Casey, the American commander in Iraq, and other military officers.

Rumsfeld's trip comes as US military commanders contemplate reducing the number of American troops in the country in the coming months.

There are about 132,000 American troops in Iraq at present.

Rumsfeld indicated earlier this week that the Pentagon intended to reduce the size of the US military presence, but gave no specific numbers nor a timetable.

Opinion polls suggest that US public support for the three-year-old Iraq war is decreasing, which contributes to a drop in President George Bush's job approval ratings.

Rumsfeld himself has been criticised by six retired generals who have demanded his dismissal, accusing him of disregarding military advice, ruling by intimidation and making strategic errors.

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