BOGOTA (Reuters) - At least 40 soldiers and 10 peasants were reported to have died when Marxist rebels launched a new bid to storm the mountain stronghold of Colombia‘s top right-wing death squad chieftain, authorities said late on Tuesday. About 500 Revolutionary Armed Forces of Colombia (FARC) guerrillas attacked four hamlets in northern Cordoba province Monday, killing 10 peasants and razing a number of homes, according to Mario Carrascal, mayor of Puerto Libertad, a town close to the combat zone. The heaviest clashes occurred late on Tuesday when army troops poured into the area to hunt down the rebel unit. A spokesman for the army‘s 11th Brigade said at least 40 soldiers were feared to have died in a FARC ambush. Military sources said the FARC had been attempting to push into the Nudo de Paramillo mountain range, the power base of its most bitter enemy, Carlos Castano, head of an illegal alliance of ultra-right wing death squads known as the United Self-Defence Forces of Colombia (AUC). Castano has spearheaded a „dirty war“ against Colombia‘s estimated 20,000 guerrillas and their suspected sympathisers. If the death toll is confirmed it would be one of the heaviest defeats for the army this year. The FARC overran Castano‘s mountaintop fortress last December, killing and mutilating 30 peasants, but narrowly failing to capture Castano. It was not immediately known if Castano was involved in the latest clashes. The FARC is currently taking part in peace talks with the government in an effort to find a negotiated settlement to the country‘s three-decade-old civil conflict, which has claimed more than 35,000 lives in just 10 years. But the rebels have said the negotiations must go ahead „in the midst of war“ and have repeatedly said they will not declare a ceasefire. In a separate incident of political violence, 13 peasants were slain by unidentified gunmen in a rural area near the town of San Carlos in northwest Antioquia province on Monday.