An Iranian fighter jet has crashed, killing one of two pilots onboard, Iranian state broadcaster IRIB said.
The air force plane crashed during a late-night training exercise in western Hamadan province, IRIB posted on Telegram.
"The cause of the accident is under investigation," it added.
The incident comes amid massive US military buildup near Iran.
President Donald Trump has deployed warships, fighter jets and other military hardware to the Middle East as he seeks to block Iran from building a nuclear bomb, something Tehran says it is not pursuing.
Trump has repeatedly threatened military action against Iran, citing a deadly crackdown on protesters last month then more recently over its nuclear programme.
On Thursday, Trump said that Iran had at most 15 days to make a deal and again suggested that the United States would attack if failed to do so.
Iran in response warned that US bases, facilities and assets would be "legitimate targets" if the US follows through on its military threats and attacks.
Amir Saeid Iravani, the Iranian ambassador to the United Nations, made the remarks in a letter to UN Secretary General Antonio Guterres and the president of the Security Council.
The letter cited a social media post by Trump on Wednesday, where he said the United States may need to use UK military bases, including one on an Indian Ocean island, "should Iran decide not to make a deal."
"Such a belligerent statement by the President of the United States...signals a real risk of military aggression, the consequences of which would be catastrophic for the region and would constitute a grave threat to international peace and security," Iravani wrote in the letter.
He called for the Security Council — the UN's top decision-making body where Washington has veto power — to "ensure that the United States immediately ceases its unlawful threats of the use of force."
A previous attempt at negotiations collapsed when Israel launched surprise strikes on Iran last June, beginning a 12-day war that Washington briefly joined to bomb Iranian nuclear sites.
