WASHINGTON — The United States expects Iran will take months to live up to its end of a seven-nation nuclear pact that could eventually provide the country relief from international sanctions.

The deal formally took effect Sunday, opening the way for Iran to make major changes to an underground nuclear facility, a heavy water reactor and a site for enriching uranium.

The changes will not happen immediately, and Iran must further constrain its nuclear program before relief from sanctions will occur.

“These next steps will allow us to reach the objectives we set out to achieve over the course of nearly two years of tough, principled diplomacy,” President Obama said in a statement.

“I am confident in the extraordinary benefits to our national security and the peace and security of the world” from putting the agreement in place, Obama said.

Senior administration officials said Saturday they understand it’s in Iran’s best interest to work quickly, but they are only concerned that the work is done correctly. They insisted that no relief from the penalties will occur until the U.N.’s International Atomic Energy Agency has verified Iran’s compliance with the terms of the agreement. They said Iran’s work will almost certainly take more than the two months Iran has projected.

The administration officials spoke on a conference call with reporters, but under the condition that they not be identified by name.

As part of the nuclear agreement, Obama on Sunday issued provisional waivers and a memorandum instructing U.S. agencies to lay the groundwork for relieving sanctions on Iran.

In Iran, Ali Akbar Velayati, a top adviser to supreme leader Ayatollah Ali Khamenei told state TV: “On implementation, all should be watchful that Westerners, particularly Americans, to keep their promises.”

Velayati said Iran expects that the United States and other Western countries that negotiated the deal will show their “good will” through lifting sanctions.

Iran’s atomic energy chief, Ali Akbar Salehi, told state TV that Tehran was ready to begin taking steps to comply, and awaited an order from President Hassan Rouhani. “We are hopeful to begin in the current or next week,” he said.

The IAEA said Sunday that Iran has agreed to allow greater monitoring of its commitment to the deal, going beyond basic oversight provided by the safeguards agreement that IAEA member nations have with the agency. For instance, it allows short-notice inspections of sites the IAEA may suspect of undeclared nuclear activities.

Even as the terms of the deal begin taking effect, recent developments have shown the wide gulf between the United States and Iran on other issues.

Fighters from Iran have been working in concert with Russia in Syria, and a Revolutionary Court convicted a Washington Post reporter who has been held more than a year on charges including espionage. The court has not provided details on the verdict or sentence. Two other Americans are being detained.