US President Donald Trump threatened Iran on Sunday with bombing and secondary tariffs if Tehran did not come to an agreement with Washington over its nuclear program.

In Trump’s first remarks since Iran rejected direct negotiations with Washington last week, he told NBC News that US and Iranian officials were talking, but did not elaborate.

“If they don’t make a deal, there will be bombing,” Trump said in a telephone interview. “It will be bombing the likes of which they have never seen before.”

“There’s a chance that if they don’t make a deal, that I will do secondary tariffs on them like I did four years ago,” he added.

Iran sent a response through Oman to a letter from Trump urging Tehran to reach a new nuclear deal, saying its policy was to not engage in direct negotiations with the United States while under its maximum pressure campaign and military threats, Tehran’s foreign minister was quoted as saying on Thursday.

Iranian President Masoud Pezeshkian reiterated the policy on Sunday. “Direct negotiations (with the US) have been rejected, but Iran has always been involved in indirect negotiations, and now too, the Supreme Leader has emphasized that indirect negotiations can still continue,” he said, referring to Ayatollah Ali Khamenei.

In the NBC interview, Trump also threatened so-called secondary tariffs, which affect buyers of a country’s goods, on both Russia and Iran. He signed an executive order last week authorizing such tariffs on buyers of Venezuelan oil.

Speaking to reporters later on Sunday on Air Force One, Trump said he is going to make a decision on the secondary tariffs based on whether Tehran makes a deal.

“We’ll probably give it a couple of weeks and if we don’t see any progress, we’re going to put them on. We’re not putting them on right now. But if you remember, I did that six years ago, and it worked very well,” he said.

In his first 2017-21 term, Trump withdrew the US from a 2015 deal between Iran and world powers that placed strict limits on Tehran’s disputed nuclear activities in exchange for sanctions relief.

Trump also reimposed sweeping US sanctions. Since then, the Islamic Republic has far surpassed the agreed limits in its escalating program of uranium enrichment.

Tehran has so far rebuffed Trump’s warning to make a deal or face military consequences.

Western powers accuse Iran of having a clandestine agenda to develop nuclear weapons capability by enriching uranium to a high level of fissile purity, above what they say is justifiable for a civilian atomic energy program.

Tehran says its nuclear program is wholly for civilian energy purposes.