Iran has detained an Iranian-American consultant working for George Soros' Open Society Institute, the latest US citizen connected to a non-governmental organization to be seized in the country, the institute said Wednesday. The detention of Kian Tajbakhsh comes amid a crackdown on NGOs at a time when Iranian authorities accuse the United States of using critics and dissidents to overthrow the country's hard-line government. Tajbakhsh, an urban planning consultant who has also worked for the World Bank, was detained on or about May 11, the Open Society Institute - a private foundation that encourages democracy-building - said in a statement. "We are concerned for his safety and call for his immediate release," the statement said. It said the 45-year-old Tajbakhsh was working as a consultant for the insitute since 2004 "with the knowledge of the Iranian government."