Edging toward a historic compromise, the U.S. and Iran reported progress Monday on a deal that would clamp down on Tehran’s nuclear activities for at least 10 years but then slowly ease restrictions on programs that could be used to make atomic arms.

Officials said there were still obstacles to overcome before a March 31 deadline, and any deal will face harsh opposition in both countries. It also would be sure to further strain already-tense U.S. relations with Israel, whose leaders oppose any agreement that doesn’t end Iran’s nuclear ambitions.

Israeli Prime Minister Benjamin Netanyahu is expected to strongly criticize the deal in an address before Congress next week.

Still, a comprehensive pact could ease 35 years of U.S.-Iranian enmity — and seems within reach for the first time in more than a decade of negotiations.

“We made progress,” Secretary of State John Kerry said as he bade farewell to members of the American delegation at the table with Iran.

More discussions between Iran and the other six nations involved in the talks were set for next Monday, a senior U.S. official said.

Iranian Foreign Minister Mohammad Javad Zarif said the sides found “a better understanding” at the negotiating table.

Western officials familiar with the talks cited movement but also described the discussions as a moving target, meaning changes in any one area would have repercussions for other parts of the negotiation.

The core idea would be to reward Iran for good behavior over the last years of any agreement, gradually lifting constraints on its uranium enrichment and slowly easing economic sanctions.

Iran says it does not want nuclear arms and needs enrichment only for energy, medical and scientific purposes, but the U.S. fears Tehran could re-engineer the program to produce the fissile core of a nuclear weapon.

The U.S. initially sought restrictions lasting up to 20 years; Iran has pushed for less than a decade. The prospective deal appears to be somewhere in the middle.

One variation being discussed would place at least a 10-year regime of strict controls on Iran’s uranium enrichment. If Iran complies, the restrictions will be gradually lifted over the final five years.

One issue critics are certain to focus on: Once the deal expires, Iran could theoretically ramp up enrichment to whatever level it wanted.

Experts say Iran already could produce the equivalent of one weapon’s worth of enriched uranium with its 10,000 currently operating centrifuges. Several officials spoke of 6,500 centrifuges as a potential point of compromise, with the U.S. trying to restrict them to Iran’s mainstay IR-1 model instead of more advanced machines.

Under a possible agreement, Iran also would be forced to ship out most of the enriched uranium it produced or change it to a form that would be difficult to convert for weapons use. It takes about one ton of low-enriched uranium to process into a nuclear weapon, and officials said that Tehran could be restricted to an enriched stockpile of no more than about 700 pounds.

Israel was already weighing in.

Defense Minister Moshe Yaalon warned that such a deal would represent “a great danger” to the Western world and said it “will allow Iran to become a nuclear threshold state.”