Iran and the International Atomic Energy Agency (IAEA) said Saturday that they've agreed a roadmap to resolve issues crucial in efforts to revive the 2015 nuclear deal.

"We have agreed to provide the IAEA by the end of (the Iranian month of) Khordad (June 21) with documents related to outstanding questions between Tehran and the agency," Iran's nuclear chief Mohammad Eslami told a joint news conference on Saturday with IAEA chief Rafael Grossi.

Grossi arrived in Tehran late on Friday to discuss one of the last thorny issues blocking the revival of the pact. The deal limits Iran's enrichment of uranium, making it harder for Tehran to develop material for nuclear weapons, in return for the lifting of economic sanctions.

"It is important to have this understanding ... to work together, to work very intensively," Grossi told the televised news conference. "Without resolving these (remaining outstanding) issues, efforts to revive the Joint Comprehensive Plan of Action (JCPOA) may not be possible."

"There are still matters that need to be addressed by Iran," Grossi added.

Soon after, Russia said that before it backs the deal it would seek written guarantees from the U.S. that sanctions on Russia would not damage its cooperation with Iran under the 2015 nuclear deal.

"We have asked for a written guarantee ... that the current process triggered by the United States does not in any way damage our right to free and full trade, economic and investment cooperation and military-technical cooperation with the Islamic Republic," Russia's Foreign Minister Sergei Lavrov told reporters.

The remaining issues, according to Iran's Foreign Ministry spokesperson Saeed Khatibzadeh, are the extent to which sanctions would be rolled back, providing guarantees that the United States will not quit the pact again, and resolving questions over uranium traces found at several old but undeclared sites in Iran.

The UK said on Friday that a deal between Tehran and world powers on Iran's nuclear program is "close."

"We are close. E3 negotiators leaving Vienna briefly to update ministers on state of play," British diplomat Stephanie Al-Qaq said on Twitter, referring to negotiators from Britain, France and Germany.

Tehran and Washington have held more than 11 months of indirect talks in Vienna on reviving the pact, which limited Iran's enrichment of uranium to make it harder for Tehran to develop material for nuclear weapons, in return for a lifting of economic sanctions.

(With input from agencies)