Iran has deactivated thousands of its centrifuges, authorities in Tehran have said, but the U.N. body conducting its mission in the country as part of the nuclear deal is still investigating allegations that the country has engaged in nuclear weapons research. Iran will not enact the final implementation of the nuclear deal until the International Atomic Energy Agency, the U.N. body, closes its investigation for good, Iranian Foreign Minister Mohammad Javad Zarif said Sunday. The announcement will likely delay the lifting of sanctions, a process that is supposed to allow companies from America to begin trading with the country.

“We have always announced that the issues raised about the Iranian nuclear program’s past lack validity,” Zarif said Sunday at a news conference in Tehran. “Closure of the ['Possible Military Dimensions'] case is necessary for continuation of the JCPOA [Joint Comprehensive Plan of Action] implementation,” he said of the sanctions relief for nuclear restrictions deal reached in July between Iran and six major powers, including the United States.

The Iran nuclear deal went into motion this summer and the IAEA began overseeing Iran’s scaling back of its nuclear program, which consisted largely of deactivating centrifuges. U.S. and European companies then began making plans to begin trading, drafting contracts for future transactions.

But American companies, or companies owned by Americans, are not allowed to begin trading with Iran until "Implementation Day," a day designated in the nuclear agreement that marks the official lifting of sanctions and the beginning of trade. On Implementation Day, the U.N., European Union and U.S. will all lift certain nuclear-related sanctions against Iran. 

Iran Gas Iranian leaders and businessmen are eagerly awaiting the lifting of sanctions on the country. Pictured: The Phase 4 and Phase 5 gas refineries are seen in Assalouyeh, 1,000 km (621 miles) south of Tehran, Jan. 27, 2011. Photo: Reuters/Caren Firouz

But that day will only come when the IAEA verifies publicly that Iran has completed all of the required steps agreed upon in the nuclear deal. Until then, companies cannot trade with Iran, except for a limited number of sectors. Some restrictions will remain in place after Implementation Day. For instance, the supply of certain goods and technology relating to missiles.

Under the terms of the ongoing sanctions, Iran is barred from purchasing certain commodities such as aircraft from U.S. and European entities -- and U.S. and European entities are barred from selling them to Iran -- until Tehran has satisfied inspectors with the IAEA that it has begun rolling back its nuclear program.

Despite the restriction on trade before Implementation Day, some companies already have begun trading, violating U.S. and EU sanction law.

One of Iran’s commercial airlines last week bought a U.K.-manufactured jet with the aim of using it to deliver Iranian soldiers and weapons to Syria in support of the embattled regime of President Bashar Assad, International Business Times recently learned.

Most U.S. individuals will still be prohibited from trading with Iran even after the lifting of sanctions. They must first obtain a license from the Office of Foreign Assets Control in the Department of the Treasury. 

Iran parliament The Iranian parliament Tuesday passed a bill approving the deal on the country’s nuclear program with the six world powers. Pictured: Iranian MPs listen to the speech of Foreign Minister Mohammad Javad Zarif in the parliament in Tehran, July 21, 2015. Photo: Getty Images/AFP/Behrouz Mehri

Individuals and companies not from the U.S., though, will be able to, at least in part, engage almost immediately in trade deals with Iran that target some of the country’s most profitable sectors. That means that the easing of sanctions under the nuclear deal will initially help businesses in EU member states, not those in the U.S., lawyers say.

Foreign governments, companies and banks are already in talks with Iran ahead of Implementation Day. 

It is still unclear when the parties of the nuclear agreement will announce the timing of Implementation Day. U.S. officials have said it could be anytime in January, but if Iran refuses to move forward without the closing of the IAEA probe, that day could be pushed back, delaying trade.

"We hope that the next report of Yukiya Amano [IAEA's director general] will help reflect the realities and put an end to this issue," Press TV quoted Zarif as saying Sunday.