The Argentine navy is stepping up its search in the South Atlantic for a 44-crew submarine that has been out of radio contact for three days. President Mauricio Macri said all national and international resources were being deployed to help find the San Juan as quickly as possible. A Nasa research plane has joined the search for the vessel. <br/><br/>Navy official said, they have not been able to find, or have visual or radar communication with the submarine. The diesel-electric submarine was returning from a routine mission to Ushuaia, near the southern-most tip of South America, to its base at Mar del Plata, south of Buenos Aires. Its last contact with the navy command was on Wednesday morning.<br/><br/>An Argentine destroyer and two corvettes are conducting a search around the area of the sub's last known position off the south-eastern Valdez peninsula. But so far there are no clues about its whereabouts.