Colombian rebels kill 15 soldiers

President Uribe has launched a massive offensive against the Farc

At least 15 soldiers have been killed and 10 wounded in a series of clashes with Farc left-wing rebels, Colombian army officials said.

In the worst incident, a patrol was ambushed in a jungle and lost 12 men as it fought its way out.

Observers fear the attacks herald an escalating offensive leading up to the presidential election in May.

The Farc are opposed to President Alvaro Uribe, who has vowed to wipe them out with military force.

The rebels - Revolutionary Armed Forces of Colombia - have vowed all-out opposition to the re-election of President Uribe, who has changed the Colombian constitution to allow him to stand for a second term.

The BBC's Jeremy McDermott in Medellin says the guerrillas are unlikely to prevent an Uribe victory, but the US-backed president is facing a deteriorating situation of public order.

Mr Uribe launched a massive offensive against the Farc, known as Plan Patriot, but sources within the defence ministry admit it has stalled and the guerrillas have re-taken the initiative, our correspondent says.