Iranian authorities will soon send a reply to US President Donald Trump's letter to the Islamic Republic's Supreme Leader, Ayatollah Ali Khamenei, Iranian foreign minister Abbas Araghchi has said.

"We [i.e. Iran] will definitely not hold direct negotiations under conditions of increased pressure, threats, and sanctions. Negotiations should be conducted on equal terms. We will reply to Trump's letter in the coming days and send it through appropriate channels," Araghchi said on the air of Iranian state television and radio.

He noted that Trump's letter "contains threats."

"But it also notes that there are opportunities. We will see both the threats and the opportunities," added the Iranian foreign minister.