Donald Rumsfeld, US defence secretary, said on Tuesday that the US would seek to "drain the swamp" inhabited by terrorists, repeating his threat that Afghanistan was not the only country that faced possible US military strikes in Washington's war against terrorism.

Speaking at the Pentagon, Mr Rumsfeld said tackling terrorism meant also fighting those who harboured terrorists.

Terrorist networks had no armed forces nor high-value targets for the US to attack. But, he said: "Those countries that support them and give sanctuary do have such targets. The terrorists do not function in a vacuum."

Mr Rumsfeld hesitated when asked if he had evidence of state support for the attacks.

He said he would leave such questions to the Department of Justice and intelligence agencies. "I know a lot, and what I have said as clearly as I know how, is states are supporting these people," he said.

Mr Rumsfeld was the latest member of the administration to prepare Americans for the likelihood that the war against terrorism would be long.

It would be fought through the political, economic, financial and intelligence networks as well as with military means. US officials made clear on Tuesday that economic sanctions were a weapon under consideration.

"It's not a matter of a single event. We're talking about a very broadly based campaign to go after the terrorist problem where it exists, and it exists in countries across the globe," he said.

The Al-Qaeda network of Osama bin Laden, the chief suspect behind the September 11 attacks, may have activities in 50 to 60 countries, including the US.

"Our adversaries are not one or two terrorist leaders, or even a single terrorist organisation or network," he said.

The government has been keeping its military planning under unusually tight secrecy, though Mr Rumsfeld and others have acknowledged the difficulties the military would face in operating in Afghanistan, where Mr bin Laden is thought to have taken refuge.

Mr Rumsfeld also said there were no plans to call up more reservists to help fill out the country's defences.

The Pentagon has already ordered 35,000 reservists be called up and Mr Bush has signed an order authorising the call-up of as many as 50,000 troops.