US-US President Donald Trump has said the United States was monitoring Iranian demonstrations closely, warning against any new “massacre” as protests broke out after Iran admitted to shooting down a passenger plane.

Iran said earlier it unintentionally shot down a Ukrainian jetliner outside Tehran, killing all 176 people aboard, in an abrupt about-turn after initially denying claims it was struck by a missile. The firing came shortly after Iran launched missiles at bases in Iraq housing American forces.

Iranian President Hassan Rouhani said a military probe into the tragedy had found “missiles fired due to human error” brought down the Boeing 737, calling it an “unforgivable mistake.”

At a student protest to pay tribute to the victims on Saturday, Iranian authorities briefly detained Britain’s ambassador, in what the British government called a violation of international law. He was later released.

Trump told Iranians - in tweets both in English and Farsi - that he stands by them and is monitoring the demonstrations.

“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,” he tweeted. “There can not be another massacre of peaceful protesters, nor an internet shutdown. The world is watching,” he added, apparently referring to an Iranian crackdown on street protests that broke out in November.

“We are following your protests closely, and are inspired by your courage,” he said.

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. There can not be another massacre of peaceful protesters, nor an internet shutdown. The world is watching. The new demonstrations follow an Iranian crackdown on street protests that broke out in November. Amnesty International has said it left more than 300 people dead.

Internet access was reportedly cut off in multiple Iranian provinces ahead of memorials planned a month after the protests.

Canadian Prime Minister Justin Trudeau has demanded that Iran provide “full clarity” on the downing of the plane. Ottawa says the dead included 57 Canadians.

Iran’s Supreme Leader Ayatollah Ali Khamenei also offered his condolences and ordered the armed forces to address “shortcomings” so that such a disaster does not happen again.

Iran’s acknowledgement came after officials in Iran denied for days Western claims that the Ukraine International Airlines plane had been struck by a missile in a catastrophic error.

The Kiev-bound jet slammed into a field shortly after taking off from Tehran’s Imam Khomeini International Airport on Wednesday. The crash came hours after Iran launched missiles at bases hosting American forces in Iraq in response to the killing of top Iranian general Qasem Soleimani in a US drone strike. Fears grew of an all-out war between Iran and its arch-enemy the United States, but those concerns have subsided after Trump said Tehran appeared to be standing down after targeting the US bases.

On Saturday evening, police dispersed students who had converged on Amir Kabir University in Tehran to pay tribute to the victims, after some among the hundreds gathered shouted “destructive” slogans,

Fars news agency said.