​​​​​​WASHINGTON

President Donald Trump said Iran is “in big trouble” as protests spread across the country, saying the US is closely monitoring developments and warning Iranian authorities against using lethal force.

“Iran's in big trouble. It looks to me that the people are taking over certain cities that nobody thought were really possible just a few weeks ago,” Trump told reporters at the White House. “We’re watching the situation very carefully.”

Noting his previous warning to Tehran, he said, “If they start killing people like they have in the past, we will get involved.”

“This is something pretty incredible that's happening in Iran. It's an amazing thing to watch,” he said. “They've done a bad job. They've treated their people very badly, and now they're being paid back.”

Trump added that any US response would not involve troops on the ground but would focus on measures that would “hit them very hard where it hurts.”

Since late December, Iran has seen waves of protests because of a steep decline in the value of the Iranian rial and deteriorating economic conditions. The demonstrations started Dec. 28 near Tehran's Grand Bazaar and later expanded to several cities. The country has been offline for more than 12 hours, according to internet watchdog NetBlocks.

Iranian officials have not released official casualty statistics. On Thursday, however, the Human Rights Activists News Agency said 2,277 people had been arrested, dozens injured, and 42 killed, including eight security personnel.

Iran has accused the US and Israel of fomenting the unrest, with officials warning Friday that security forces and the judiciary "will show no tolerance whatsoever toward saboteurs."