President Hugo Chavez of Venezuela has threatened to halt oil exports to the US if his country is attacked by Colombia – a close ally of US. The threat comes amid an escalating dispute over allegations that Venezuela is harbouring Colombian rebels. Mr Chavez broke diplomatic ties with Colombia last week and put his army on high alert. Venezuela is America's fifth biggest source of imported oil, supplying about a million barrels a day. Mr Chavez said he had received intelligence that “the possibility of armed aggression against Venezuela from Colombia was higher than it had ever been”. The Venezuelan leader also said he had cancelled a trip to Cuba to celebrate a revolutionary anniversary with his close ally, President Raul Castro, because of the danger of attack.