Iran says it has no knowledge of the whereabouts of retired Federal Bureau of Investigation agent Robert Levinson, who US authorities have long presumed was captured there. In a statement to The Associated Press, the Iranian press counsellor at the United Nations, Alireza Miryusefi, called on the US government to explain Levinson's mission in Iranian territory, after an AP investigation revealed that he had been on an unauthorised assignment for the CIA when he vanished on Iran's Kish Island in March 2007. "Actually the American authority should explain about Levinson's mission in Iranian soil," Miryusefi told the news service. "They are responsible and should respond." He insisted that Tehran had been co-operating with attempts to learn Levinson's fate. The Iranian government has been trying "to find any clue about Levinson's situation for humanitarian and security reasons, but no success", he said. US officials have raised the Levinson case with Iran repeatedly over the years, but until the AP investigation last week it was not known that Levinson was hoping to gather information in his role as an independent contract investigator who expected to be compensated by a group of analysts at the Central Intelligence Agency. The last photos and video of a bearded, emaciated Levinson were released anonymously to his family in 2010 and early 2011, and investigators say his trail has grown cold since. Levinson, who would be 65, has been missing longer than any other presumed US hostage.