() Iran will deal out a “strong blow” against the United States if attacked, the country’s Supreme Leader Ayatollah Ali Khamenei said Monday, according to Iranian state media.

The threat comes days after President Donald Trump warned Iran he would bomb their country should they not agree to a nuclear weapons deal to restrain the country’s nuclear program. The Middle Eastern country said Sunday it rejected direct negotiations with the U.S.

“If they don’t make a deal, there will be bombing, and it will be bombing the likes of which they have never seen before,” Trump said in a comment aired Sunday on NBC News.

But, Iran’s president says the country has no plans to directly negotiate with the U.S., writing, “We don’t avoid talks; it’s the breach of promises that has caused issues for us so far. They must prove that they can build trust.” 

Trump also threatens secondary tariffs on Iranian oil

Trump also warned that an imposition of “secondary tariffs” on Iranian oil would be possible, meaning that countries that buy Iranian oil would be faced with U.S. tariffs.

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

Trump’s first term: US pulled out of Iran nuclear program deal

During his first term, Trump pulled the U.S. out of a deal that put strict limits on Iran’s nuclear program in exchange for sanctions relief.

The United Nations’ nuclear watchdog has since raised alarms about Iran developing nuclear weapons.

The president is attempting to curtail Iran’s rapidly advancing nuclear program, issuing a letter that arrived in Iran on March 12.

What did Trump’s letter to Iran say?

NBC News reported that Trump’s letter urged Iran to enter negotiations on the nuclear program, offering a two-month window for Iranian leaders to make a decision. Iran did respond to the letter, with Iranian President Masoud Pezeshkian explaining that Tehran would not enter negotiations but would talk indirectly.

According to Politico, Pezeshkian said, “Although the possibility of direct negotiations between the two sides has been rejected in this response, it has been emphasized that the path for indirect negotiations remains open.”

It is not clear if Trump will accept these indirect negotiations.

The International Atomic Energy Agency reported earlier this year that Tehran has accelerated its production of near-weapons-grade uranium, though Iran insists its program is for peaceful purposes.