US President Donald Trump said the United States would react strongly if authorities in Iran started executing people in their crackdown on a popular uprising against the government.
"We will take very strong action if they do such a thing," he told CBS News in an interview, when asked about executions potentially beginning today.
In a video clip released online, he said: "When they start killing thousands of people - and now you're telling me about hanging.
"We'll see how that's going to work out for them."
The interview took place as Mr Trump was in Michigan to visit a manufacturing plant and deliver a speech on the economy.
In his speech, the US president reiterated a message that he had posted earlier on social media in which he said "help is on its way" for the Iranian protesters.
Asked what he meant by "help is on its way", Mr Trump told reporters they would have to figure that out.
Mr Trump has said military action is among the options he is weighing to punish Iran over the crackdown.
The unrest, sparked by dire economic conditions, has posed the biggest internal challenge to Iran's rulers for at least three years and has come at a time of intensifying international pressure on Iran over its nuclear and ballistic missiles programmes.
Mr Trump said he cancelled all meetings with Iranian officials until the "senseless killing" of protesters stopped and in a later comment told Iranians to "save the name of the killers and the abusers ... because they'll pay a very big price".
Iran has accused Mr Trump of encouraging political destabilisation and inciting violence alongside threatening the country's sovereignty, territorial integrity and national security in a letter from Iran's UN Ambassador Amir-Saeid Iravani to the UN Security Council.
"The United States and the Israeli regime bear direct and undeniable legal responsibility for the resulting loss of innocent civilian lives, particularly among the youth," he wrote in the letter, which was also sent to UN Secretary-General Antonio Guterres.
He wrote the letter in response to a social media post by Mr Trump.
Meanwhile, Iran warned US allies in the Middle East it will strike US bases on their soil if the US attacks Iran, a senior Iranian official said.
The Iranian official, speaking on condition of anonymity, said Iran had asked US allies in the region to "prevent Washington from attacking Iran".
"Tehran has told regional countries, from Saudi Arabia and UAE to Turkey, that U.S. bases in those countries will be attacked" if the US targets Iran, the official said.
The official added that direct contacts between Iran's Foreign Minister Abbas Araqchi and US Special Envoy Steve Witkoff had been suspended, reflecting mounting tensions.
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The US president said it was unclear what the death toll in Iran actually was.
"I hear numbers - look, one death is too much - but I hear much lower numbers and then I hear much higher numbers," he said.
Later, speaking to reporters on his return to Washington, Mr Trump said he would soon be receiving a briefing on Iran.
"The killing looks like it's significant, but we don't know yet for certain," the president said.
Mr Trump had previously vowed the United States would get involved if protesters were killed, a line crossed days ago.
The US-based HRANA rights group said it had verified the deaths of 2,571 people during Iran's protests, including 2,403 protesters, 147 government-affiliated individuals, 12 people aged under 18 and nine non-protester civilians.
An Iranian official said yesterday that about 2,000 people had been killed in the protests, the first time authorities have acknowledged the high death toll from an intense crackdown on two weeks of nationwide unrest.
The Iranian official said that people he called terrorists were behind the deaths of both protesters and security personnel.
The official, who declined to be named, did not give a breakdown of who had been killed.
Iran's chief justice Gholamhossein Mohseni Ejei said, during a visit to one of Tehran prisons where people arrested in protests are being held, that speed in punishing those "who beheaded or burned people" was critical to ensure that such events would not take place again.
Fears have also grown that the Islamic republic could use the death penalty to crackdown on the protests, after Tehran prosecutors said authorities would press capital charges of "moharebeh", meaning waging war against God, against some suspects arrested over recent demonstrations.
"Concerns are mounting that authorities will once again resort to swift trials and arbitrary executions to crush and deter dissent," Amnesty International said.
The Norway-based NGO Iran Human Rights highlighted the case of 26-year-old Erfan Soltani, who was arrested last week in the Tehran satellite city of Karaj and who, according to a family source, has already been sentenced to death and is due to be executed as early as today.
