Iranians gather in celebration in northern Tehran on July 14, 2015, after Iran's nuclear negotiating team struck a deal with world powers in Vienna (AFP Photo/Atta Kenare)

Washington (AFP) - More than 100 former US ambassadors including the ranking diplomat taken hostage in Tehran in 1979 came out Thursday in support of President Barack Obama's nuclear deal with Iran.

In a joint open letter, former hostage Bruce Laingen and a host of ex-envoys praised the "landmark agreement."

"If properly implemented," they said, the deal, which trades international sanctions relief for nuclear curbs, "can be an effective instrument in arresting Iran's nuclear program."

It could prevent "the spread of nuclear weapons in the volatile and vitally important region of the Middle East," the signatories said.

Among those who signed on to the statement are former ambassador to NATO Nicholas Burns, former ambassador to Iraq and Afghanistan Ryan Crocker and former UN ambassador Tom Pickering.

But the list also includes Laingen, who was the charge d'affaires -- the head of mission -- in Tehran when students stormed the US embassy.

Laingen was held with 51 other Americans for 444 days, a crisis that scuttled Jimmy Carter's presidency and put relations between Iran and the United States in deep freeze for three decades.

Laingen's wife, Penelope, tied a yellow ribbon around an oak tree at their home near Washington, an act replicated across the country and which became emblematic of a country in crisis.

Obama came to office vowing to talk to Tehran's leaders, still in power 35 years after the Islamic Revolution.

But he has been accused of appeasing a regime that backs US-designated terror groups across the Middle East.

Those in favor and against the deal have launched a fierce lobbying campaign ahead of an expected Congressional vote that could leave it dead in the water.