Trump warns Iran against 'another massacre of peaceful protesters'

"The government of Iran must allow human rights groups to monitor and report facts from the ground on the ongoing protests by the Iranian people," Trump tweeted.

His message came after Iranian police dispersed students chanting "radical" slogans, according to Fars news agency, during a Saturday gathering in Tehran to honour those killed when the Ukraine International Airlines plane was shot down shortly after taking off from Tehran.

AFP correspondents said hundreds of students had gathered early in the evening at Amirkabir University in downtown Tehran, to pay respects to those killed in the air disaster. The tribute later turned into an angry demonstration.

Videos shared on social media show protesters chanting "Commander-in-chief resign, resign" - referring to Supreme Leader Ayatollah Ali Khamenei - "death to liars", and "death to the dictator".

"To the brave, long-suffering people of Iran: I've stood with you since the beginning of my Presidency, and my Administration will continue to stand with you," Trump tweeted in both English and Farsi.

"We are following your protests closely, and are inspired by your courage," he added.

The new demonstrations follow a bloody crackdown on nationwide protests that broke out in November. As many as 1,500 people were killed by security forces in just two weeks.

"There can not be another massacre of peaceful protesters, nor an internet shutdown. The world is watching," Trump said.

According to media reports, internet access was effectively cut off in multiple Iranian provinces ahead of memorials planned a month after the protests.

Trump's comments come amid a time of heightened tensions between Washington and Tehran.

Just hours before the Ukrainian passenger jet was shot down, Iran fired a volley of ballistic missiles at two Iraqi bases hosting US troops. That attack came in retaliation for the killing of top Iranian general Qasem Soleimani in a US drone strike last week.

The shock strike many express fears of war in the region, with President Trump saying the US would target 52 sites, including a number "cultural sites", if Iran attacked US assets again.

The US has also called on Tehran to apologise for the detention of the British ambassador to Tehran, reportedly during the protests outside Amirkabir University.

"This violates the Vienna Convention, which the regime has a notorious history of violating. We call on the regime to formally apologize to the UK for violating his rights and to respect the rights of all diplomats," State Department spokeswoman Morgan Ortagus tweeted.

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