Iran heralded an “historic” agreement about its nuclear program with great powers as a “first step” toward better relations with the rest of the world on Friday. President Hassan Rouhani said he hoped for broader rapprochement with the West that could end more than 35 years of tension.

The words came after Iranian negotiators reached a framework agreement with representatives of the United Nations Security Council’s five permanent members plus Germany in Switzerland on Thursday.

According to the agreement, Iran will cut the number of centrifuges that enrich uranium and reduce its existing uranium stockpile both by two-thirds. Iran would not build new facilities and open up its nuclear sites to inspections. If it complies with the terms, Western powers will gradually lift sanctions on its oil-based economy.

American officials said the deal would extend the “breakout time” needed for Iran to make a bomb to a full year, from two or three months now.

Iran has always denied it intended to develop nuclear weapons, insisting its program was peaceful. Western powers found that hard to believe, given the history of Iranian deception and hostility since the 1979 revolution replaced the monarchy with the incumbent theocratic Shia regime.

Ties with the United States have been especially hostile. After Shah Mohammad Reza Pahlavi, its ally, was deposed, America backed the Arab Sunni powers in the region who see Iran as their rival for hegemony.

President Barack Obama, who faces skepticism about his willingness to do a deal with Iran from opposition Republicans and American allies in the Middle East, including Israel, said on Thursday the United States should seize the chance for an “historic understanding” with its rival. If fully implemented, he said, the accord “will prevent it from obtaining a nuclear weapon.”

Obama emphasized that the work was “not yet done” and warned the Iranians against backsliding. If they don’t fully meet the inspections and verifications regime, the president said, “there will be no deal.”

He also dampened hopes for a full reparation of ties. As long as Iran “continues its sponsorship of terrorism, its support for proxies who destabilize the Middle East, its threats against America’s friends and allies, like Israel,” America will remain vigilant, Obama said.

France, which has taken a hard line in the negotiations, was also cautious. Laurent Fabius, the foreign miniser, said, “We are not completely at the end of the road and the end of the road should be in June.” That is when a permanent agreement with Iran should be reached.