US Defense Secretary Donald Rumsfeld warned Iraq's new Shiite elite not to purge the fledgling security forces and to beware of corruption as he made a surprise visit to Baghdad on Tuesday.

The US embassy, meanwhile, scrambled to find out more about an American contractor kidnapped from a reconstruction site around noon Monday in the Baghdad area, an embassy spokesman said.

Washington is concerned Iraq's new leaders will "come in and clean house," with newly trained security forces, Rumsfeld told reporters on the flight from Washington to Baghdad.

"You can't do that, if you are trying to create a chain of command in the Iraqi security force and defeat a doggone insurgency," said Rumsfeld, whose country disbanded Iraq's army after the US-led invasion of March 2003.

"Anything they do in the interior and the defense ministries ought to be with an eye to the fact that Iraqis are getting killed, and they better have a good reason for doing what they are doing."

The comment was the bluntest to date by a US official over fears that Iraq's election-winning Shiite political bloc aimed to carry out mass purges of the security apparatus and other ministries.

Rumsfeld told reporters that he would be meeting President Jalal Talabani and prime minister-designate Ibrahim Jaafari and military commanders during his one-day visit.

He was to become the most senior American official to meet the top two representatives of Iraq's new government, which is still being formed more than two months after January 30 national elections.

Rumsfeld also warned against a major overhaul of ministerial staff as the price of political patronage.

"We have an opportunity to continue to make progress politically, economically ... Anything that would delay that or disrupt that as a result of turbulence, or lack of confidence or corruption in government, would be unfortunate," he said.

Such mass turnovers were a problem under the previous transitions from Iraq's Governing Council to an interim government under ex-prime minister Iyad Allawi, Rumsfeld said.

US diplomats have privately told Iraqi politicians that it would be disastrous to expel members of the new security forces, a US official told reporters.

He said Washington was walking a fine line to avoid the appearance of meddling in Iraq's political affairs and tainting the process for the general public.

Jaafari is expected to form a full government in the next two weeks, with the crucial post of interior minister going to Jaafari's religious Shiite coalition, the United Iraqi Alliance (UIA), which has the largest share of parliament seats.

UIA members have been calling for a purge of Iraq's security forces, particularly the interior ministry and national intelligence service.

The UIA claims those two branches have been over-run by former Baathists who were recruited by Allawi, himself a former party member who fell out with Saddam Hussein.

One of the largest Shiite parties, the Supreme Council for the Islamic Revolution in Iraq (SCIRI), is expected to take the interior ministry portfolio and wants to integrate its own Iranian-trained Badr militia organisation into the security forces.

Much of the leadership of elite Iraqi units now deployed in insurgent hotspots across central Iraq, including Fallujah, Samarra, Mosul and Ramadi, are made up of former Baathists. Ironically, many of them are Shiite.

Washington fears the removal of these veterans, who have now proven their stripes on the battlefield, could set America back in the war against Iraq's insurgency.

Attacks continued against Iraqi forces on Tuesday.

Insurgents shot dead a bodyguard in an attack in Baghdad on Iraqi army General Tareq Baldawi and wounded two others, an interior ministry source told AFP.

Ex-Baath Party official Mahmoud Jassem Shaji and two of his relatives were killed overnight by armed men near Baquba, about 140 kilometers (85 miles) northeast of Baghdad, an Iraq army officer said.

An army patrol found the three bodies.

A number of ex-Baath Party officials have died in revenge killings since US forces toppled Saddam's regime two years ago.

On Monday, three US marines and three Iraqis were wounded when three suicide car bombers blew themselves up outside a US military base in a volatile area of Iraq near the Syrian border.

A US official said the attack was part of a new strategy by the insurgency's Islamist extremists to try to rally Sunni Arabs with a dramatic strike against the Americans and to deter them from joining the political process.