Rights groups voiced alarm Tuesday over the extent of an Iranian crackdown on a Kurdish-populated city that has become a hub for protests, as oil refinery workers pressed strikes in a new tactic.

Iran's clerical authorities have been shaken by over three weeks of protests that erupted after the death of Mahsa Amini, a 22-year-old Iranian of Kurdish descent, who had been arrested by the notorious morality police.

Despite the use of brutal force by the authorities that activists say has left dozens dead, and led to hundreds of arrests, there is so far no sign of the protest movement coming to an end.

Protests have been especially intense in the city of Sanandaj in the western province of Kurdistan, Amini's home region, where rights groups fear heavy casualties and accuse authorities of resorting to the shelling of neighborhoods.

The Norway-based Hengaw rights groups said an Iranian warplane had arrived at the city's airport overnight and buses carrying special forces were on their way to the city from elsewhere in Iran.

It warned locals were having problems sending video evidence of events due to internet restrictions but said a 7-year-old had been killed on Sunday night. Agence France-Presse (AFP) could not immediately verify the claims.

Amnesty International said it was "alarmed by the crackdown on protests in Sanandaj amid reports of security forces using firearms and firing tear gas indiscriminately, including into people's homes."

"Iranian authorities continue to disrupt internet and mobile networks to hide their crimes," it said in a statement.

All together!

The New York-based Center for Human Rights in Iran said there was a risk of a similar situation in the Sistan-Baluchistan province in the southeast where activists say over 90 people have been killed since Sept. 30.

"The ruthless killings of civilians by security forces in Kurdistan province, on the heels of the massacre in Sistan-Baluchistan province, are likely preludes to severe state violence to come," said its director Hadi Ghaemi.

Analysts have said these protests are proving particularly challenging for the authorities under Supreme Leader Ayatollah Ali Khamenei, 83, due to their duration and multi-faceted nature ranging from street demonstrations to individual acts of defiance.

In a new development on Monday, protests spread to Iran's oil refineries with videos showing striking workers burning tires and blocking roads outside the Asalouyeh petrochemical plant in the southwest.

They could also be heard shouting slogans including "Death to the dictator" and "Don't be scared, we are all together!"

Similar actions were reported at other facilities including Abadan in the west where the Oslo-based NGO Iran Human Rights (IHR) said a strike was already in progress on Tuesday.

University campuses and even school classrooms have also seen regular protests, with students at the Amir Kabir University of Technology in Tehran on Monday shown chanting anti-regime slogans.

In a video shared by the 1500tasvir social media channel that monitors protests and police violations, students at the Tehran Art University were shown spelling out the Persian word for blood – "Khun" – in a human chain.

Vain effort to silence

The crackdown on the protests sparked by Amini's death on Sept. 16 has claimed at least 95 lives, according to the IHR.

Activists say that among those who died in the protests are two teenage girls, Nika Shakarami and Sarina Esmailzadeh, whose families say were killed by security forces after being detained. Authorities insist they died in falls.

Another 90 people were killed by the security forces in Iran's far southeastern city of Zahedan on Sept. 30 after the protests sparked by the alleged rape of a teenage girl by a police chief in the Sistan-Baluchistan province, said the IHR, citing the U.K.-based Baluch Activists Campaign.

UNICEF Executive Director Catherine Russel said, "We are extremely concerned by continuing reports of children and adolescents being killed, injured and detained amid the ongoing public unrest in Iran."

The crackdown has prompted international condemnation with U.S. National Security Advisor Jake Sullivan telling Iran the "world is watching" and "will hold responsible those using violence in a vain effort to silence" protesters.

The U.K. on Monday said it had imposed sanctions on Iran's morality police, the unit which arrested Amini and enforces strict dress rules for women including the compulsory headscarf.

Iran said it has summoned the British ambassador to protest against the "baseless" sanctions.

French Foreign Minister Catherine Colonna said on Tuesday that five French citizens are currently being held in Iran, in comments implying that one of the nine foreigners Iran has said it is holding over links to the protests is French.