The FBI said on Tuesday it was "encouraged" by Iran's offer to help locate Robert Levinson, a former FBI agent who went missing on the Iranian island of Kish in March 2007.
"We are encouraged by the Government of Iran's offer to work with the FBI in an effort to locate Mr. Levinson and return him home to his family safely," said Joseph Persichini, assistant director of the FBI's Washington field office.
"We immediately responded to that offer accepting their assistance and continue to await a response," he added, noting that the investigation was ongoing.
"We are working closely with the US Department of State to facilitate communication with the Government of Iran. We continue to update the Levinson family in this humanitarian effort."
State Department acting spokesman Robert Wood said "no information has been forthcoming" from Iran.
"What we have been doing for quite some time now is to call on the Iranians to do more -- provide more information with regard to Mr. Levinson," he said.
"So again, we call on the Iranians to provide whatever information they have on Mr. Levinson so that we can, hopefully, return this gentleman to his family."
In Senate testimony during a confirmation hearing in mid-January, Secretary of State Hillary Clinton called on Iran to release Levinson.
"It would be an extraordinary opportunity for the government of Iran to make such a gesture, to permit contact, to release him, to make it clear that there is a new attitude in Iran, as we believe there will be with the Obama administration, towards engagement," she said.
Christine Levinson says her husband, who retired from service a decade ago, had traveled to Kish island to investigate cigarette counterfeiting in the region and was last heard of on March 8.
"I believe he is alive and he's being held by the Iranians," Florida Senator Bill Nelson told Newsweek. Nelson represents the family of Levinson, a Floridian.
Nelson unveiled Tuesday a resolution with Florida Representative Robert Wexler and Levinson family members that "urges the President and the allies of the United States to engage with officials of the Government of Iran to raise the case of Robert Levinson at every opportunity," Nelson said in a statement.
"It is critical that the United States government use every diplomatic tool at our disposal and raise his case at the highest levels internationally and with the Iranian government," said Wexler.
Newsweek has reported that "some US intelligence officials" said Tehran may consider releasing Levinson as part of a prisoner swap for several Iranian diplomats seized by US military forces in northern Iraq.
The mystery of Levinson's disappearance is a further strain in relations between the United States and Iran, which have had no diplomatic ties for nearly three decades and remain at loggerheads over the Iranian nuclear drive.
The White House, which has ordered a review of US policy toward Tehran, pledged on Tuesday that the United States will use "all elements of our national power to deal with Iran" after Tehran said it had launched a satellite into orbit.
The administration of President Barack Obama said it planned to raise the issues of Iran's nuclear and missile programs at talks in Germany on Wednesday with European allies, Russia and China.
In an interview last week, Obama said the United States would offer Iran an extended hand of diplomacy if the Islamic Republic's leaders "unclenched their fist."