Amnesty International said there were reports that security forces had used firearms indiscriminately in Sanandaj.

Kurdish group Hengaw posted a video which it said showed police shooting at homes in the city and another in which gunfire and cries could be heard.

It reported that at least five civilians had been killed and 400 injured across the region since Sunday.

But it warned that the death toll might be higher because authorities were disrupting local internet and mobile networks.

Protests against the clerical establishment have swept across Iran since the death three weeks ago of Mahsa Amini, a Kurdish woman from the western city of Saqqez who fell into a coma after being detained by morality police in Tehran for allegedly violating the strict hijab law.

The unrest is now considered the most serious challenge to the Islamic Republic since its inception in 1979.

Iran's leaders have accused foreign enemies and exiled opposition groups of fomenting "riots" that they will not tolerate.

Hengaw reported on Tuesday that over the past three days protests had taken place in 10 areas of Kurdistan, Kermanshah and West Azerbaijan provinces, with Sanandaj the epicentre of the unrest and the crackdown by authorities.

The Norway-based group posted videos which it said showed intense clashes between protesters and security forces in the city on Monday night. Repeated gunfire can be heard in the footage, as well as cries and shouts.

According to Hengaw, other footage showed a crowd coming under attack, empty bullet cases, shotgun cartridges and tear-gas canisters left on the streets, and security personnel shooting directly towards homes.

Videos of ongoing demonstrations by schoolgirls in Sanandaj and elsewhere in the region were also posted online.

In one, more than a dozen girls are seen waving their headscarves in the air in the middle of a road and shouting "woman, life, freedom", which is the main slogan of protests triggered by Mahsa Amini's death.

Another in Saqqez showed a crowd shouting "death to the dictator" - a reference to the Supreme Leader, Ayatollah Ali Khamenei.

During a visit to the city on Tuesday, Interior Minister Ahmad Vahidi blamed "counter-revolutionary terrorist" groups for the "riots" and vowed that security forces would continue to respond with an iron hand, state media said.

At least 185 people, including 19 children, have been killed by security forces since the protests erupted on 17 September, according to Iran Human Rights, another Norway-based group.

The government has denied killing protesters and said that more than 20 security personnel have died.

Thousands of people have also reportedly been arrested in the crackdown.

They include the daughter of late former president Akbar Hashemi Rafsanjani, Faezeh Hashemi, who the judiciary said on Tuesday had been charged with "disruption of public order and propaganda against the Islamic Republic", according to AFP news agency.

In another development, protests by workers in Iran's vital oil and gas industry reportedly widened on Tuesday.

Videos appeared to show people demonstrating outside the Abadan oil refinery in Khuzestan province, Iran's oldest and largest in terms of output.

"This is the beginning of the road and we continue our protests every day, together with people all over the country," the Wall Street Journal cited a committee representing contract oil workers as saying in a statement.

However, state media denied the reports of a strike.

On Monday, dozens of employees blocked roads outside the Assaluyeh petrochemical complex in Bushehr province and shouted: "Do not fear, we stand together." A regional official insisted it was related to a wages dispute.