World powers were set to open a fifth round of talks Tuesday with Iran aimed at bringing the United States back into the landmark 2015 nuclear deal meant to prevent the Islamic Republic from obtaining an atomic bomb.

The talks in Vienna come the day after the U.N.’s nuclear watchdog, the International Atomic Energy Agency, was able to strike a last-minute agreement with Tehran on a one-month extension to a deal on surveillance cameras at Iran’s nuclear sites. The issue wasn’t directly related to the ongoing talks on the nuclear deal, known as the Joint Comprehensive Plan of Action, or JCPOA, but had Iran not agreed it could have seriously complicated the discussions.

The U.S. is not directly involved in the talks but an American delegation headed by President Joe Biden’s special envoy for Iran, Rob Malley, has also been in the Austrian capital. Representatives from the other powers involved — Germany, France, Britain, Russia and China — have been shuttling between the U.S. and the Iranians to facilitate indirect talks.

As he headed back to Vienna for the resumption of talks, Malley tweeted that the latest round had been “constructive and saw meaningful progress.”

“But much work still needs to be done,” he wrote. “On our way to Vienna for a fifth round where we hope we can further advance toward a mutual return to compliance.”

Russia’s delegate, Mikhail Ulyanov, who has consistently been the most optimistic about the possibility of an agreement, suggested a resolution was in sight.

“I think it can be final,” he tweeted about the fifth round. “But in order to be on the safe side I would prefer to say: let’s see.”

In 2018, then-President Donald Trump pulled the U.S. unilaterally out of the agreement saying that it was not broad enough and needed to be renegotiated. As part of a “maximum pressure” campaign, he reimposed sanctions and added additional ones on Iran in an effort to bring Tehran back to the table.

Iran’s economy was crippled by the move but it has refused new talks, instead retaliating by slowly and steadily breaking the restrictions of the JCPOA in an effort to pressure the other parties involved, thus far unsuccessfully, to come up with incentives to offset the American sanctions.

Biden, who was vice president when the original deal was negotiated, has said he wants the U.S. to rejoin but that Iran has to return to complete compliance. Iran has insisted that all American sanctions imposed under Trump be dropped, including measures that were taken in response to non-nuclear issues.

Iran’s violations include a significant increase in the purity and quantity of uranium it has been enriching, effectively reducing the so-called break out time to produce an atomic bomb, even though that is something Iran says it does not want to do, insisting that its nuclear program is for civilian purposes only.

On Monday, Ali Akbar Salehi, the head of Iran’s civilian program, said Tehran had stockpiled 5 tons of uranium enriched up to 5% purity, 90 kilograms (198 pounds) enriched up to 20% and 2.5 kilograms (5.5 pounds) up to 60% — still below weapons-grade levels of 90% purity.

Though the West fears it could be used to help Tehran potentially obtain an atomic bomb, U.S. intelligence agencies have said they “assess that Iran is not currently undertaking the key nuclear weapons-development activities … necessary to produce a nuclear device.”

Despite Iran’s violations of the JCPOA, the other nations involved have stressed that the agreement was still important as it allowed IAEA inspectors to continue their surveillance of Iran’s nuclear facilities.

Continuity of that surveillance was threatened, however, until the last-minute agreement negotiated by the IAEA with Iran on Monday.

Under a confidential agreement called an “Additional Protocol” with Iran, the IAEA collects and analyzes images from a series of surveillance cameras installed at Iranian nuclear sites. Those cameras helped it monitor Tehran’s program to see if it is complying with the nuclear deal.

Iran’s hard-line parliament in December approved a bill that would suspend part of U.N. inspections of its nuclear facilities if European signatories didn’t provide relief from oil and banking sanctions by February.

The IAEA then struck a three-month deal with Iran in February to have it hold the surveillance images, with Tehran threatening to delete them afterward if no deal had been reached.

Speaking at a news conference Monday in Vienna as the deal was due to expire, IAEA Director-General Rafael Grossi told journalists Iran had agreed to a one month extension though his agency still cannot access the images taken by the cameras for the time being.

“I’d want to stress this is not ideal,” Grossi said. “This is like an emergency device that we came up with in order for us to continue having these monitoring activities.”

Kazem Gharibabadi, Iran’s representative to the IAEA, acknowledged the deal at the same time on Twitter. He said Tehran’s civilian nuclear agency, the Atomic Energy Organization of Iran, would keep the material already recorded by the IAEA cameras.

“We recommend the negotiating countries to seize the extra opportunity provided by Iran in good faith for the complete lifting of sanctions in a practical and verifiable manner,” Gharibabadi wrote.

(AP)