"The decision of the new US government to exert pressure on the Iranian nation by preventing Iran's legal trade with its economic partners is an illegitimate, illegal and violative measure," foreign ministry spokesman Esmaeil Baqaei said in a statement.

The US move was "categorically unjustified and contrary to international rules", Baqaei added.

The US Department of the Treasury on Thursday announced financial sanctions against an international network "facilitating the shipment of millions of barrels of Iranian crude oil worth hundreds of millions of dollars" to China.

The oil was shipped on behalf of Iran's Armed Forces General Staff and a sanctioned front company called Sepehr Energy Jahan Nama Pars, the treasury department wrote in a statement.

The sanctions came after US President Donald Trump reinstated his "maximum pressure" policy against Iran over allegations the country is seeking to develop nuclear weapons.

Iran has slammed the reinstatement of the policy -- similar to one Trump levied against Tehran during his first term as president -- saying pursuing it would end in "failure".

Under the tough policy of sanctions during Trump's first term, which ended in 2021, Washington withdrew from a landmark nuclear deal that had imposed curbs on Iran's nuclear programme in return for sanctions relief.

Tehran adhered to the deal -- known as the Joint Comprehensive Plan of Action -- until a year after Washington pulled out, but then began rolling back its commitments.

Efforts to revive the 2015 deal have since faltered.

Trump on Wednesday called for a "verified nuclear peace agreement" with Iran, adding that it "cannot have a Nuclear Weapon".

Iran insists its nuclear programme is solely for peaceful purposes and denies any intention to develop atomic weapons.