A historic comprehensive agreement has been reached over the Iranian nuclear issue, a diplomatic source confirmed to Xinhua on Tuesday.

The agreement was clinched between Iran and the six major world countries -- Britain, China, France, Germany, Russia and the United States.

Representatives of these countries held a final plenary at the UN headquarters here before making an announcement of the deal.

"I believe this deal is a historic moment, a new chapter of hope," Iranian Foreign Minister was quoted as saying by televised news conference.

Meanwhile, Yukiya Amano, director general of the International Atomic Energy Agency (IAEA), said he has signed a roadmap with Iran to clarify past and present outstanding issues, according to media reports.

"I have just signed the roadmap between the Islamic Republic of Iran and the IAEA for the clarification of past and present outstanding issues regarding Iran's nuclear program," Amano said ahead of the expected announcement of the historic deal.

Future access to Iran's Parchin military site, which the agency had repeatedly sought, is part of a separate "arrangement", Amano said.