Betancourt is a severe critic of the FARC

Mr Pastrana last week ordered the army to retake the rebels' enclave - a Switzerland-size area ceded to them three years ago - after they hijacked an airliner and took a senator hostage.

The United States has added its voice to international condemnation of the Revolutionary Armed Forces of Colombia (FARC) and called for Mrs Betancourt's immediate release.

She was seized on Saturday as she travelled into the rebels' former enclave.

Click here for a map of the former FARC safe haven

An army general says the whereabouts of the rebels who kidnapped Mrs Betancourt are known, but a rescue operation has been cancelled after her family "asked that her life not be endangered".

Kidnapping is a favoured tactic of the guerrillas, who are believed to be holding about 800 people hostage.

'Typical and tragic'

"Kidnapping members of Congress, now kidnapping a presidential candidate, and kidnapping Colombians is kidnapping democracy," President Pastrana said in the southern city of Neiva.

Troops are moving into the rebel's former haven

Mrs Betancourt's abduction was "a very tragic example" of typical FARC tactics, said State Department spokesman Richard Boucher in Washington.

The US backed Mr Pastrana's decision to end peace talks with the guerrillas, he said.

The European Union and the United Nations have also condemned the rebels' actions.

Mrs Betancourt, a severe critic of the FARC, is running as an independent in the poll on 26 May. She was trailing last in opinion polls.

She and her campaign manager were seized as they travelled towards San Vicente - about 250 km (170 miles) south of the capital, Bogota - to attend a human rights rally on Saturday.

Government officials condemned the FARC but also branded Mrs Betancourt irresponsible for making the trip, after being advised that the road was not safe.

President Pastrana has said there will be no further peace talks until the rebels stop using terror tactics.

But the BBC's Peter Greste in Colombia says any talk of negotiations now is academic.

With the presidential elections only three months away, he says, nothing is likely to happen until a new national leader is in place.