"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," he added.
The tweets show Trump's solidarity with the thousands of people who gathered in front of the gate of the Amirkabir University of Technology, near the former US embassy in Tehran, to protest Iran's Supreme Leader Ayatollah Ali Khamenei, for unintentionally downing a civilian passenger plane and killing 176 people.
Iran admitted on Saturday that its Revolutionary Guard Air defense forces mistakenly brought down Ukrainian Airlines Flight 752 shortly after it took off from Tehran on January 8.
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• Ukrainian plane crash in Iran: Missile strike or terrorism possible causes
• Iran says it 'unintentionally' shot down Ukrainian plane
• Iran plane crash: Expert questions what caused plane to crash
• Iran plane crash: 63 Canadians and three Britons among 176 dead after plane crashed near Tehran
"Armed Forces internal investigation has concluded that regrettably missiles fired due to human error caused the horrific crash of the Ukrainian plane & death of 176 innocent people," President Hassan Rouhani said on Twitter.
"Investigations continue to identify and prosecute this great tragedy and unforgivable mistake," he added.