I have given all the relevant orders to the armed forces to retake the area

President Andres Pastrana

The move came after guerrillas hijacked a commercial aircraft and seized a prominent senator.

Mr Pastrana made a national TV address and gave the rebels just a few hours' notice of the offensive.

"I have decided not to continue the peace process with the FARC. I have decided to put an end to the demilitarized zone, beginning at midnight," he said.

In their response, the FARC said: "With this rupture, the government shows yet again it is dedicated to war."

Mr Pastrana has spent most of his three-and-a-half years in office trying to broker peace deals with Colombia's rebel groups. The policy has lost popular backing, and he almost called off the talks in January because progress was so slow.

He said that, after the kidnapping of opposition senator Jorge Gechen Turbay on Wednesday, "the glass of indignation spilled over".

The hijackers, who the government said were members of the FARC, took over a Dash-8 Turboprop belonging to the airline AIRES shortly after it took off from Neiva, the capital of Huila department, en route for the capital Bogota.

The kidnappers blew up a bridge behind them

The plane was forced to land on a road near the town of Hobo, 27 miles (45km) south of Neiva, where the hijackers bundled Mr Turbay and possibly another unidentified passenger into a vehicle and sped away.

"There were 50 or 70 guerrillas waiting for us on the ground when we landed," one male passenger told local television.

He and the other 30 or so passengers and crew were left on the road, unharmed.

The escaping hijackers blew up a bridge behind them as they headed towards the FARC enclave, the army said.

Senator Gechen Turbay comes from one of Colombia's most prominent political families, members of which have been kidnapped and killed in the past by the FARC.

There were 50 or 70 guerrillas waiting for us on the ground

Passenger

But a spokesman for the FARC, Raul Reyes, said he did not know whether his group was responsible.

FARC has been responsible other hijackings, including a flight to Bogota in January 2001 with 31 people on board, and another incident in September 2000 when a rebel forced a flight from Neiva to take him to the guerrilla safe haven.

In a significant hardening of his position, President Pastrana said he was authorising the army to take offensive as well as defensive action when clearing the rebels from the safe haven they were granted in late 1998 as an incentive to enter meaningful peace talks.

However, he stressed the army should take "special care of the civilian population."

He said he was reissuing arrest warrants for FARC leaders, and annulling the group's political status.