Human Rights Watch (HRW) on Wednesday (November 27th) accused the Iranian authorities of "deliberately covering up" deaths and arrests during a crackdown on demonstrations across the country earlier this month.
Protests broke out across Iran on November 15th, hours after a shock announcement of fuel price rise of up to 200%.
Reports of deaths and arrests emerged as security forces were deployed to rein in demonstrations, which turned violent in some areas.
But the extent of the crackdown is unclear, primarily due to an internet outage imposed during the unrest in a step seen as aimed at curbing the spread of videos of the violence.
HRW said the authorities were "deliberately covering up the scale of the mass crackdown against protestors".
The rights watchdog called on them to "immediately announce the number of deaths, arrests, and detentions... and permit an independent inquiry into alleged abuses".
Its deputy Middle East director, Michael Page, censured Iran for having so far "refused to provide an accurate death toll and instead threatened detainees with death".
Rights groups, including Amnesty International, have estimated at least 140 people were killed and up to 7,000 arrested in the protests, HRW said in a statement.
'Government strategy to stifle dissent'
Officials have confirmed five people were killed and have so far announced the arrest of about 500 others, including some 180 "ringleaders".
"Keeping families in the dark about the fate of their loved ones while ratcheting up an atmosphere of fear and retribution is a deliberate government strategy to stifle dissent," Page said.
Internet connectivity has returned to much of the country in recent days, except for mobile telephone networks, said NetBlocks, a site that monitors internet disruptions.
The US said Tuesday it has received nearly 20,000 messages from Iran about protests including photos and videos after appealing to demonstrators to defy restrictions on the internet.
Secretary of State Mike Pompeo had earlier asked Iranians to send in information so the US can "expose and sanction the abuses".
"We have received to date nearly 20,000 messages, videos, pictures, notes of the regime's abuses through Telegram messaging services," Pompeo said.
"To the courageous people of Iran who refuse to stay silent about 40 years of abuses by the ruling regime, I say simply this: the US hears you, we support you and will continue to stand with you in your struggle for a brighter future for your people and for your great nation," he said.
In response to Iran's effort to shut down the internet, the US has imposed sanctions on the communications minister and pressed Facebook, Twitter and Instagram to suspend the accounts of Iranian government leaders.