U.S. 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, Reuters reported.

In Trump's first remarks since Iran rejected direct negotiations with Washington last week, he told NBC News that U.S. 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, according to Reuters.

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.