Secretary of State John F. Kerry meets with Federica Mogherini, E.U. high representative for foreign affairs and security policy, on July 2 at a hotel in Vienna where the Iran nuclear talks are being held. (Carlos Barria/AFP/Getty Images)

Iran has agreed to a procedure that would allow international inspectors to gain some access to its nuclear facilities before sanctions are lifted, a senior Iranian official said Thursday. It was the strongest indication yet that a final deal to curb Tehran’s nuclear program may be days away.

“We have nothing to hide,” said the official, a negotiator who spoke on the condition of anonymity in a rare briefing for Western reporters.

The complicated process he outlined is consistent with what U.S. officials at the negotiations have said in recent days, and it appears to differ from a more hard-line position that Iran’s supreme leader, Ayatollah Ali Khamenei, staked out last month.

It is based on the principle of “simultaneity,” officials said. Once an agreement is reached, its provisions would not immediately take effect. Instead, the United States and the European Union would each lay the groundwork for lifting sanctions, a time-consuming task that involves drafting regulations and notifying banks, among other measures.

At the same time, the Iranian official said, Tehran would provisionally agree to abide by the Advanced Protocol — the voluntary amendments to the nuclear Non-Proliferation Treaty that most nuclear nations follow — until the Iranian parliament ratifies it.

That would allow inspectors from the International Atomic Energy Agency (IAEA) to negotiate “managed access” to declared and undeclared Iranian nuclear and nuclear-related facilities, including at least some access to those connected to military sites.

“We are not prepared to allow our military secrets to be shared,” the official said. “By accepting the Advanced Protocol, we are not meaning that all of our military complexes are open for the inspectors to go wherever they want, to see whomever they want, to collect information wherever they want. We are talking about managed access, which is the usual process with the agency.”

The United States and the five world powers negotiating with Iran have insisted on assurances that Tehran is not engaged in any covert nuclear activity before sanctions are lifted.

Iran is seeking guarantees that the sanctions, once lifted, will not resume unless it is determined by an agreed procedure that the country has not upheld its commitments. Likewise, the Iranian official said, if the United States and Europeans do not keep their commitments under a deal, “Iran has the right to go back to its [nuclear] program as it wishes.”

“What is important is that both sides are treated more or less equally,” he said.

Even as the official was speaking, Yukiya Amano, the head of the IAEA, was in Iran, where he met with Iranian President Hassan Rouhani to clarify some of the inspection and verification issues in the talks, according to news reports from Tehran. Amano also met there with Ali Shamkhani, secretary of the Supreme National Security Council, according to news reports from Tehran.

“Iran, as in the past, is ready to continue collaborating closely with the IAEA to resolve the remaining issues,” Shamkhani told local reporters.

The pace at which international sanctions would be lifted as Iran meets commitments and the depth and breadth of international verification of its compliance are the two primary issues at the talks.

Foreign ministers from world powers negotiating with Iran began arriving here Thursday, joining Secretary of State John F. Kerry for a final push to reach a deal by an extended deadline set for Tuesday. Kerry met again with his Iranian counterpart Thursday evening.

Earlier Thursday, British Foreign Secretary Philip Hammond said the talks had not reached a “breakthrough moment.”

“You’re going to see over the next few days ministers coming and going to maintain the momentum of these discussions,” he said.

The negotiating partners are the five permanent members of the U.N. Security Council, plus Germany.

Iranian Foreign Minister Mohammad Javad Zarif, who had traveled to Tehran for one day of consultations this week, said he will stay in Vienna until the end of the talks, the state-run Islamic Republic News Agency reported.

In a meeting with reporters here earlier in the week, a senior Obama administration official described a “pause” between agreement and implementation. “We have worked out a process and a phasing so that Iran will take a series of voluntary nuclear steps that will be verified,” the official said. “We have a lot of preparation to do to be able to lift our sanctions in the first instance. . . . It is not something you just turn a switch and all of a sudden it’s gone.”

The official deadline for a deal was June 30, but the negotiators said Tuesday that they would extend an interim agreement for a week, in effect setting a new deadline of July 7.

That is just two days before the White House must get a final deal to Congress for a 30-day review. If the administration waits until July 10 or later, the congressional review period will grow to 60 days, which potentially could cause political problems for the administration.