Restrictions on Iranian oil exports remain in place while negotiators work toward a formal nuclear agreement, the U.S. State Department said.

The five permanent members of the U.N. Security Council, plus Germany, are working with their Iranian counterparts to formalize a framework agreement that pulls Iran back from the brink of having the technology needed to develop a nuclear weapon.

Iran is restricted to around 1 million barrels per day in exports to a few consuming nations. That's about half the level from before a joint arrangement went into force in late 2013.

Marie Harf, a spokeswoman for the State Department, said it was premature to consider what would happen if the framework agreement is finalized later this year.

"The restrictions on oil purchases remain in place," she said during a Tuesday press briefing. "So what will happen under a comprehensive joint plan of action, I just don't know yet."

Iranian Oil Minister Bijan Zangeneh said in early April the Islamic republic could return as a dominant force in the Organization of Petroleum Exporting Countries once the nuclear deal is formalized. Iran holds the third-largest proven oil reserves among OPEC member states, behind Venezuela and Saudi Arabia, respectively.

Petroleum officials in Iran said they expect foreign investors to "flock to Iran" in the wake of any sanctions relief. Zangeneh traveled last week to China, one of Iran's top oil customers, to discuss a potential increase in oil sales.