Iran's Revolutionary Guards deploy forces to three provinces to put down an eruption of anti-government unrest while Trump says the US would throw its support behind those protesting in Iran at an "appropriate time."
UN Secretary-General Antonio Guterres in a statement issued on Wednesday deplored the loss of life in Iran protests and called for further violence to be avoided.
Earlier during the day, the UN human rights chief also called on Iran to rein in security forces to avoid further violence as tens of thousands gathered across Iranian cities in a show of massive support for the government.
Zeid Ra'ad al-Hussein, UN High Commissioner for Human Rights, in a statement urged the government to respect the right of protesters to freedoms of expression and peaceful assembly.
Protesters "have a right to be heard," he said.
There must be "a concerted effort by the authorities to ensure that all security forces respond in a manner that is proportionate and strictly necessary, and fully in line with international law".
He said more than 20 had been killed and hundreds arrested across Iran in the past week and urged "thorough, independent and impartial investigations of all acts of violence that have taken place".
TRT World'sKim Vinnell reports.
Iran deploys Revolutionary Guards
Iran's elite Revolutionary Guards have deployed forces to three provinces to put down an eruption of anti-government unrest, their commander said on Wednesday, after six days of protests that have left 21 people dead.
The Revolutionary Guards commander, Major General Mohammad Ali Jafari, said he had dispatched forces to Isfahan, Lorestan and Hamadan provinces to tackle "the new sedition".
Most of the casualties among protesters have occurred in those regions.
The Revolutionary Guards, the sword and shield of Iran's Shia theocracy, were instrumental in suppressing the 2009 uprising, killing dozens of protesters then.
Three members of Iran's intelligence forces killed in the western city of Piranshahr, in a fight with "anti-revolutionaries", the Mehr news agency reported.
Tens of thousands gathered in cities across Iran on Wednesday in a massive show of support for the government after days of deadly unrest, state television showed.
Crowds chanted "Leader, we are ready" as images showed vast numbers marching through the cities of Ahvaz, Kermanshah, Gorgan and elsewhere.
The crowds waved Iranian flags and pictures of supreme leader Ayatollah Ali Khamenei, as well as placards reading "Death to seditionists".
"We offer the blood in our veins to our leader," was another popular chant.
The rallies followed several days of unrest - initially sparked by protests over economic problems but quickly turning against the government as a whole - in which 21 people lost their lives and hundreds were arrested.
Khamenei blamed foreign "enemies" for the demonstrations in a speech on Tuesday.
Trump promises support to protesters
US President Donald Trump on Wednesday said the US would throw its support behind those protesting in Iran at an "appropriate time," his latest in a string of tweets since anti-government protests erupted last week against Tehran.
"Such respect for the people of Iran as they try to take back their corrupt government. You will see great support from the United States at the appropriate time!" Trump wrote in a post on Twitter.
Such respect for the people of Iran as they try to take back their corrupt government. You will see great support from the United States at the appropriate time!— Donald J. Trump (@realDonaldTrump) January 3, 2018
Germany says protesters deserve respect
Germany said it was following developments in Iran with concern but stressed that protesters there demonstrating against economic hardship deserved respect.
"The federal government considers it legitimate when people courageously protest their economic and political woes on the streets as is happening in Iran at the moment and they have our respect," said government spokeswoman Ulrike Demmer.
She urged Iranian authorities to initiate dialogue with the protesters and to respond proportionately to demonstrators who engage in violence, adding that Germany was alarmed by reports of deaths in six days of unrest.