Under the 2015 deal, Iran shipped roughly 97 percent of its nuclear fuel stockpile out of the country, and experts do not believe it has enough on hand to produce a weapon. Ever since the United States withdrew from the agreement, Iran has sought to walk a fine line between abandoning the deal and continuing to sell its oil to foreign buyers to buoy its struggling economy.

But last month, the Trump administration announced it would no longer suspend economic penalties against eight nations that were continuing to buy Iranian oil, including China, Japan and India. And in an interview in New York 10 days ago, Iran’s foreign minister, Mohammed Javad Zarif, said he was “under pressure every day” to abandon the deal, as Mr. Trump did.

By themselves, an acceleration of work on high-speed centrifuges and limits on international inspections would not necessarily take Iran closer to producing a weapon.

But Fars on Monday quoted Ali Akbar Salehi, the M.I.T.-educated head of Iran’s Atomic Energy Organization and a key negotiator in the 2015 deal, as saying Tehran could disregard the limit “whenever we wish, and would do the enrichment at any volume and level.”

The sanctions on Iran’s oil exports were escalated two weeks after the country’s Islamic Revolutionary Guards Corps was placed on the State Department’s list of foreign terrorist organizations — the first time that the designation was given to an arm of another nation’s government.

American intelligence and Defense Department officials had opposed the terror designation, concerned that Iran would similarly target or attack American troops and intelligence operatives in the region. Last week, President Hassan Rouhani of Iran declared all American forces in the Middle East as terrorists and labeled the United States government a state sponsor of terrorism.

Three senior American officials said new intelligence that surfaced over the weekend raised concerns about the Revolutionary Guards and their activities in Iraq, where they have helped train Shiite Arab militias. The officials would not provide specific details about the threat posed by Iranian forces or Iraqi Shiite militias with ties to Tehran’s military.