Iran said Sunday it could quit a treaty against the spread of nuclear weapons after the US tightens sanctions, while an Iranian general said the US Navy was interacting as before with an elite military unit blacklisted by Washington.

Tensions between Tehran and Washington have risen since the Trump administration withdrew last year from a 2015 international nuclear deal with Iran and began ratcheting up sanctions.

Earlier this month, the US blacklisted Iran's elite Revolutionary Guards (IRGC) and demanded buyers of Iranian oil stop purchases by May or face sanctions.

"The Islamic Republic's choices are numerous, and the country's authorities are considering them ... and leaving NPT [nuclear Non-Proliferation Treaty] is one of them," state broadcaster IRIB's website quoted Foreign Minister Mohammad Javad Zarif as saying.

Iran has threatened in the past to leave the NPT, as US President Donald Trump moved to scrap the 2015 deal with world powers - the US, Russia, China, Germany, Britain and France.

Separately, Iran's armed forces chief of staff said the IRGC - which ensures security in Gulf waters and the Strait of Hormuz for Iran - had not observed any change in the US military's behavior towards the elite force after the blacklisting.

"US warships are obliged to respond to the IRGC on the passage of the Strait of Hormuz ... and until yesterday they have been answering IRGC questions, and we have not seen change in their procedures," Major General Mohammad Baqeri was quoted as saying by the semiofficial Fars news agency.

Lieutenant Chloe Morgan, US Naval Forces Central Command spokeswoman, said Sunday, "The Strait of Hormuz is an international waterway. Threats to close the strait impact the international community and undermine the free flow of commerce.

"The US, along with our allies and partners, is committed to freedom of navigation and remains well positioned and postured to preserve the free flow of commerce, and we are prepared to respond to any acts of aggression," Morgan said in an emailed statement, without referring to interaction with IRGC forces.

On Wednesday, Zarif called the IRGC blacklisting "absurd," but suggested Iran did not plan to respond militarily unless the US changed the rules of engagement guiding how it interacts with Iran's forces. The US military has not suggested it would alter its behavior ­after the blacklisting.