Iran has mobilised all its resources to sell oil in a "grey market", bypassing US sanctions that Tehran sees as illegitimate, state media quoted deputy oil minister Amir Hossein Zamaninia as saying on Sunday.

The United States, which last year withdrew from a 2015 Iran nuclear deal with world powers, has told buyers of Iranian oil to stop purchases by May 1 or face sanctions.

Iran says it will continue to export oil in defiance of US sanctions, part of a campaign by Washington aimed at halting Tehran's ballistic missile programme and curbing its regional power.

"We have mobilised all of the country's resources and are selling oil in the 'grey market'," state news agency IRNA quoted Zamaninia as saying.

Zamaninia gave no details about the "grey market", but Iran is widely reported to have sold oil at steep discounts and often through private firms during sanctions earlier this decade.

"We certainly won't sell 2.5 million barrels per day as under the (nuclear deal)," Zamaninia said, giving no figures for current sales.

"We will need to make serious decisions about our financial and economic management, and the government is working on that."

"This is not smuggling. This is countering sanctions which we do not see as just or legitimate," Zamaninia said.