WASHINGTON - US Defence Secretary Robert Gates  Wednesday called for more visits to Iran by American citizens, saying the private contact might help bridge differences between the two countries. Talking about US-Iran relations at the American Academy of Diplomacy, Gates said that "We need to figure out a way to develop some leverage with the Iranians and then sit down and talk with them." "There are actually a fair number of Iranians that come to the United States to visit," he said. "We ought to increase the flow going the other way, not of Iranians but of Americans, and that may be one way of creating some space perhaps over some period of time." The United States severed its ties with Iran in 1980. From then on, Washington has been trying to beef up its sanctions against Tehran for allegedly developing secretly nuclear weapons and involving in anti-U.S.-led coalition forces activities in Iraq. Iran flatly denies U.S. allegations. Since May 2007, the two countries have had three rounds of low-level talks over Iraq security problems in Baghdad, but failed to reach any agreement.