After more than a decade of stalemate and threats, the clock starts Monday on the six-month initial phase of a deal that negotiators for Iran, the United States and five other world powers say could lead to a permanent agreement to defuse tensions over Iran’s nuclear program.

Beginning Monday, Iran has agreed to take steps to curtail some aspects of its nuclear program in exchange for $7 billion in relief from economic sanctions that the U.S. and its allies have imposed on the Islamic republic.

The measures are intended to build confidence between the two sides that a permanent deal would be respected by both and won’t be used by the Iranians simply to get closer to building a nuclear weapon, as the U.S. has accused them of doing.

But reaching a permanent deal — and removing the threat of a military strike against Iran — is hardly a certainty. Hard-liners in both Iran and the United States oppose the interim deal, even as the Obama administration and supporters argue that it marks the first time Iran has agreed to limitations on its nuclear program.

“It’s a huge step forward,” said Daryl Kimball, executive director of the Arms Control Association, a policy institute in Washington that has closely monitored the talks. “It’s not everything we want it to be, and it’s not everything the Iranians want it to be, but it’s far better than an unconstrained Iranian nuclear program.”

Starting Monday, President Hassan Rouhani’s government is expected to halt uranium enrichment above normal fuel grade, stop adding centrifuges at enrichment plants, and end nuclear-related activities at the unfinished Arak heavy water reactor. Iran has also agreed to daily inspections from international experts.