WASHINGTON — The U.S.'s special representative for Iran Brian Hook has said that more than 1,000 Iranian citizens may have been killed in recent protests.

On Thursday, Hook told reporters that the U.S. assessment, which was higher than previous reports by news organizations and rights groups, was based on crowd sourcing and intelligence reports.

“We know for certain it is many, many hundreds,” he said.

Iran has disputed any figures on the death toll at this stage as “purely speculative” and “highly inaccurate.”

NBC News could not verify numbers of protesters killed in nationwide demonstrations that broke out on Nov. 15 in response to a 50 percent hike in gas prices.

People protest against increased gas price, on a highway in Tehran, Iran Nov. 16. Nazanin Tabatabaee/WANA / Reuters file

The U.N. High Commissioner for Human Rights Michelle Bachelet said on Friday her office had information that suggested at least 208 people were killed in the protests and had received video footage that appeared to show security forces shooting at unarmed civilians as they ran away.

"Verified video footage indicates severe violence was used against protesters, including armed members of security forces shooting from the roof of a justice department building in one city, and from helicopters in another," Bachelet said in a statement. "We have also received footage which appears to show security forces shooting unarmed demonstrators from behind while they were running away, and shooting others directly in the face and vital organs – in other words shooting to kill."

Bachelet, former president of Chile, said the incidents represented "serious violations of human rights" and expressed "alarm" at what she called Iran’s lack of transparency over casualties.

Amnesty International reported earlier this week that more than 200 people had been killed in the demonstrations.

Some Iranian experts inside the U.S. government have expressed caution about reporting on the death toll in the protests. While not denying that hundreds may have been killed, one expert noted that early counts are often inaccurate and are prone to double-counting. That caution, he said, has been relayed to U.S. policymakers.

Hook said that in addition to the hundreds allegedly killed and thousands injured, at least 7,000 protesters had been detained in Iran's prisons.

He went into detail about a reported incident in which the Islamic Revolutionary Guard Corps, an elite branch of the Iranian military, allegedly opened fire on protesters killing as many as 100 people in a Mahshahr in southwestern Iran.

The U.N. rights commissioner Bachelet said that that her office had received information "partially corroborating reports" that security forces used machine guns to fire on protesters fleeing or hiding in reed-beds in Mahshahr, leaving at least 23 dead and "possibly many more."

The New York Times first reported the incident in Mahshahr.

Hook said the U.S. officials had received videos of the incident after Secretary of State Mike Pompeo made a public request for protesters to share accounts of what was happening with the U.S. through secure channels.

Hook did not offer details about how the U.S. verified video of the incident. NBC News could not independently verify the reports.

Alireza Miryousefi, a spokesman for Iran’s mission to the United Nations, called the casualty numbers "purely speculative and highly inaccurate.”

Iranian protesters gather around a burning motorcycle during a demonstration against an increase in gasoline prices in the central city of Isfahan, on Nov. 16. AFP - Getty Images file

Miryousefi told NBC News that an investigation into what he called “disturbances” and “those affected, whether injured or killed,” was ongoing and that results would be released.

“The government will also try to compensate people for damages that occurred due to vandalism or foreign-motivated and instigated riots imposed on ordinary citizens,” he wrote in an email to NBC News.

Iran acknowledged Tuesday that its security forces shot and killed what it called "rioters" in multiple cities — the first time that authorities have offered any sort of accounting for the violence they used to put down the demonstrations, according to the Associated Press.

Iran’s Supreme Leader, Ayatollah Ali Khamenei, called Wednesday for those detained in the protests to be treated with “Islamic mercy.”

"The faster these cases are considered, the better and those who are suspected of being close to any group should be dealt with in a way that is closer to Islamic mercy," Khamenei said, according to state-run IRNA news agency.

Khamenei also said citizens killed in the protests “without playing any part in instigating them” should be considered martyrs and their families should receive government stipends.

The demonstrations began after authorities raised minimum gasoline prices to 15,000 Iranian rials per liter, an equivalent of about 50 cents a gallon. After a monthly 60-liter quota, it costs 30,000 rials a liter. That's nearly 90 cents a gallon. An average gallon of regular gas in the U.S. costs $2.58 by comparison, according to the AAA.

Widespread economic discontent has gripped the country since May last year, when President Donald Trump imposed crushing sanctions after unilaterally withdrawing from Tehran’s nuclear deal with world powers.

Tensions between Iran and the U.S. and its allies in the Gulf have been growing, with the Pentagon saying on Thursday that the U.S. was formulating plans to potentially deploy more U.S. troops to the Middle East in response to a growing threat from Tehran.

“The secretary and others are continuing to look at that threat picture and have the ability to dynamically adjust our force posture,” John Rood, defense undersecretary for policy, told senators, adding that so far no decision has been taken.

Abigail Williams reported from Washington, and Saphora Smith reported from London.