Iran's nuclear chief says the Islamic Republic's response readiness will "startle" the United States should it decide to pull out of a multilateral nuclear agreement reached between Tehran and the P5+1 group of countries in 2015.

"We hope that the other side makes a wise decision and avoids carrying out an action that would cause a nuisance for itself and others," Head of the Atomic Energy Organization of Iran (IAEA) Ali Akbar Salehi told reporters on Saturday.

Salehi once again reaffirmed Tehran's capability to resume enriching uranium to the 20-percent level in four days in response to a possible US withdrawal from the nuclear deal, officially known as the Joint Comprehensive Plan of Action (JCPOA).

He has repeatedly described the JCPOA, which was negotiated under his predecessor, Barack Obama, as “the worst and most one-sided transaction Washington has ever entered into” and threatened to tear it up.

In January, the US president decided to stick with the JCPOA, but gave the European signatories a May 12 deadline to “fix the terrible flaws” of the accord or he would withdraw Washington from the deal.

Other parties to the agreement have all criticized Trump’s hostile views, saying the deal is sound and has proven to be functioning.