Admiral Michael Mullen's remarks echo similar statements this month by NATO Secretary General Jaap de Hoop Scheffer and General David Petraeus, the commander of US forces in the Middle East.

"With respect to Afghanistan, a regional approach is critical," said Mullen, the chairman of the Joint Chiefs of Staff.

"Iran, as a bordering state, plays a role as well and to the degree that we are able to dialogue with them and find some mutual interests, there is a potential there for moving ahead together but I really leave that to the diplomats," he said.

"Iran is unhelpful in many, many ways, so I wouldn't be overly optimistic at this point but there are mutual interests that I think might offer some possibilities," he said.

The United States, which has had no diplomatic relations with Tehran for three decades, accuses Iran of seeking to develop nuclear weapons.

President Barack Obama nevertheless on Monday reaffirmed his intention to open discussions with Iran and extend a hand to the Islamic Republic if its leaders "unclenched their fist."

Mullen said the Obama administration has still not taken the military option against Iran off the table, but said "I believe it's got to be last resort."