Engaging Iran is important because Washington's military options against Tehran are unsatisfactory, said the diplomats, who worked for Republican and Democratic administrations.
The five – Colin Powell, Madeleine Albright, Warren Christopher, James Baker and Henry Kissinger – all said they favored talking to Iran as part of a strategy to stop Tehran's development of a nuclear weapons program.
"Frankly the military options here are very poor. We don't want to go down that route," said Christopher, who worked for former President Bill Clinton from 1993 to 1997.
Powell, who worked for Bush from 2001 to 2005, said US officials in Bush's first term had held low-level talks with the Iranians until 2003 "and then it was stopped."
"I agree with Madeleine, and I suspect my other colleagues, that we should try to talk to them," Powell said during a forum hosted by The George Washington University and taped for broadcast on CNN.
Albright, who was secretary of state in the second Clinton administration, had just told the group: "I believe we need to engage with Iran. I think the whole point is you try to engage and deal with countries that you have problems with."
Dealing with Iran has become an issue in the November US presidential election campaign, with Democrat Barack Obama and Republican John McCain sparring over Obama's stated readiness to talk to Iranian President Mahmoud Ahmadinejad and other US adversaries if elected president.
McCain has criticized Obama's stand, saying it shows naivete and inexperience.
The United States cut diplomatic ties with Iran in 1980, a year after an Islamic revolution toppled US-backed Shah Mohammad Reza Pahlavi and months after militant students seized the US Embassy in Tehran and held its staff hostage.
Iran has been on the US list of state sponsors of terrorism for years. Bush has been calling Iran a part of an axis of evil since 2002, and has refused to rule out using military force to stop Iran from acquiring nuclear bombs. Washington also accuses Iran of arming, financing and training Shiite militants killing US forces in Iraq.
Iran denies seeking nuclear weapons and blames the US occupation for the violence in Iraq.
In July, the Bush administration shifted tactics and sent an envoy to multilateral nuclear talks with Iran for the first time, seeking to underline Washington's stated position that it wants a diplomatic solution to the impasse.
"They would understand that, I think," Baker said.