CARACAS (Reuters) - Washington has asked the government of leftist Nicolas Maduro for access to Venezuelan-American executives of U.S.-based refiner Citgo detained in Caracas this week, a State Department official said on Thursday.       Five of six Citgo [PDVSAC.UL] executives arrested on graft allegations are U.S. citizens, a source familiar with the matter told Reuters on Wednesday. All six men are being held in the headquarters of Venezuela s military counterintelligence department in Caracas, the country s state prosecutor said in a statement on Thursday.  Socialist Maduro has said that the United States, his ideological foe, had requested the men be freed, but he vowed on Wednesday they would be tried as  corrupt, thieving traitors  for allegedly seeking to personally profit from a financial deal that was detrimental to the nation.   The U.S. Embassy in Venezuela has asked that (Venezuelan) authorities grant consular access to all U.S. citizen detainees in Venezuela. We call on the (Venezuelan government) to do so immediately in accordance with the Vienna Convention on Consular Relations,  the State Department official said.  Relations between Caracas and Washington have long been tense. They have further soured under President Donald Trump since his administration imposed sanctions on Venezuelan officials including Maduro, and economic sanctions that have impeded the OPEC nation s access to international banks. U.S.-based Citgo Petroleum Corp (Citgo) is a Venezuelan-owned refiner and marketer of oil and petrochemical products and the arrests come amid a wider anti-corruption sweep in Venezuela s oil industry. Around 50 managers at state oil company PDVSA have been arrested since August.  Sources in the energy sector say the arrests owe more to Maduro s move to sideline rivals and increase his control of money-making companies as the country struggles in a devastating recession.  The political opposition says PDVSA is rife with corruption, and a congressional investigation concluded that at least $11 billion went  missing  between 2004 and 2014.  The legacy of socialism: To have destroyed PDVSA and the oil industry and turned it into a den of corruption and nepotism,  opposition lawmaker Jose Guerra said on Twitter.  They need to blame someone and some  gringos  are ideal,  Guerra later told Reuters.  The arrests rid Citgo of much of its top brass and have instilled fear throughout Venezuela s oil industry, snarling up decision making, sources close to PDVSA have told Reuters.  The arrests also come at a highly delicate time for Venezuela, which was declared in selective default this month after some late payments. But as Venezuela is making efforts to pay, bondholders of some of the world s highest yielding debt have so far been tolerant of the delays.   