US forces are yet to find conclusive proof that Iraq possesses weapons of mass destruction, but thousands of sites remain to be searched, its top commander said on Sunday.

"I have not found any that I have absolutely satisfied myself are weapon of mass destruction materials," General Tommy Franks told CNN in an interview at the Central Command war headquarters in Qatar.

"But you're talking about 2,000, perhaps 3,000, places in this country where we know we're going to go and investigate each one of them."

The United States and Britain invaded Iraq on March 20, saying Saddam Hussein had failed to declare banned weapons.

But there has been no chemical or biological attack against invading troops, and no confirmed discovery of banned weapons has been reported.

After more than three weeks of fighting, US-led forces have arrested just one senior Iraqi official -- Saddam's top scientific adviser, General Amer Hammoudi al-Saadi, who handed himself in to the new military authorities on Saturday.

US intelligence officials believe he could be the key to unlocking secrets about Iraq's weapons programmes. But Saadi insisted Iraq had no banned chemical or biological arms.

There is speculation that senior Iraqi figures might have escaped to neighbouring Syria. Washington warned Damascus not to become a haven for Saddam's supporters or for 'terrorists'.

"Now that the regime has gone in Baghdad, we hope that Syria will understand there is an opportunity for a better way for them, if they would stop supporting terrorist activities, and we hope the Syrians will respond accordingly," US Secretary of State Colin Powell told BBC television.

But Syrian Foreign Minister Farouq al-Shara said accusations that Damascus had helped Saddam's government and provided military assistance to Baghdad were baseless.