WASHINGTON - A senior US official has said that the US sanctions on Iran will snap back, and Washington aims to cut the export of Iran's crude oil down to zero "as soon as possible".

Though Brian Hook, director of policy planning for the US State Department, said the measures to reduce market disruption are being taken, the US move on Iran still sparked concerns and skepticism.

In a news conference, Hook said the key part of US strategy on Iran is "a campaign of maximum economic and diplomatic pressure".

"The first part of our sanctions will snap back on Aug 6. These sanctions will include targeting Iran's automotive sector, trade in gold, and other key metals," he said.

"The remaining sanctions will snap back on Nov 4. These sanctions will include targeting Iran's energy sector and petroleum-related transactions, and transactions with the Central Bank of Iran."

The US focus is on "getting as many countries importing Iranian crude down to zero as soon as possible" in a bid to reduce Iran's revenue from crude oil sales, he said.

As to the banking sanctions, Washington will be aggressively enforcing these provisions to lock up Iran's assets overseas and deny Iran access to its hard currency, he said.

He noted that about 50 international firms have already announced their intention to leave the Iranian market.

Following US President Donald Trump's decision to quit the Iran nuclear pact, the United States vowed to reimpose sanctions against Teheran and inflict punishments like secondary sanctions on nations that have business links to the country.

Last month, following the US threat of new sanctions, foreign ministers and economic ministers from France, Germany and Britain urged Washington to exempt European companies engaged in trade with Teheran.

Washington's withdrawal from the deal was widely criticized and some of its major European allies have been working to prevent the 2015 accord from falling apart.

Iranian President Hassan Rouhani has warned that regional oil supply could be jeopardized if the US manages to get its allies to stop buying Teheran's oil.

Rouhani spoke to Iranian expatriates in Switzerland during his visit there on Monday. He said the US has threatened not to allow Iran to continue exporting its oil. Iranian state TV broadcast his remarks.

Dalia Dassa Kaye, a researcher at the Rand Corporation, said it was not surprising that many European nations are taking a defiant tone.

"It's certainly unrealistic to think that with the US violation of the deal rather than Iran, the United States will be able to create even stronger multilateral sanctions regimes than those that existed before the JCPOA (the nuclear deal)," Kaye said.

Xinhua - AP