Four dead in clash with Colombia's Farc ahead of talks 31 January 2013 Last updated at 17:33 Colombia's Farc rebels have stepped up their attacks, as the latest round of peace talks with the government gets under way in Cuba. Four soldiers are said to have died and two were injured in a clash with the left-wing group in southern Colombia. Meanwhile, the Farc's chief negotiator, Ivan Marquez, said he had no knowledge of the alleged kidnap of two policemen who went missing last Friday. Earlier, the rebels defended their "right" to capture "war prisoners." Police in Colombia have also blamed the Farc for the disappearance of three oil workers on Wednesday. The contractors, working for Canada's Gran Tierra Energy, were taken hostage in the southern Cauca region, news agency Reuters reported. The clashes reportedly happened as security forces tried to stop the rebels from entering the town of Policarpa, also in southern Colombia. Despite the violence, the fourth round of negotiations was set to resume in Havana on Thursday. The government's chief negotiator, Humberto de la Calle, described Farc's statement about "prisoners of war" as word play and said Colombia will not be forced into a bilateral ceasefire by the latest violence. Colombia's stance has been to only accept a bilateral truce, once a final peace agreement has been signed. After the last round of negotiations, a Farc spokesman said the negotiations were advancing at an accelerated pace - "in the rhythm of mambo," as he put it. But Humberto de la Calle has warned that "kidnapping" could harm the talks." The government says the conflict, which has lasted nearly five decades, has cost the lives of more than 500,000 people.