Colombian insurgents have killed 28 soldiers in the deadliest attack on the military since President Alvaro Uribe was elected three years ago, setting the stage for heightened tensions ahead of elections in May.

The assault by 300 rebels from the leftwing Revolutionary Armed Forces of Colombia (Farc) took place on Tuesday near the Sierra Macarena national park, in the province of Meta, south-east of Bogotá, the capital. Farc guerrillas opened fire when 80 troops were trapped in an area sown with anti-personnel mines, the military said, hampering rescue efforts.