Rights groups have sounded the alarm over an intensifying crackdown by Iranian security forces against protesters in the western province of Kurdistan, as Tehran summoned the British ambassador in response to UK sanctions against the morality police. Security forces in the provincial capital, Sanandaj, have used firearms and fired teargas “indiscriminately”, including into people’s homes, Amnesty International reported. A female protester in the city told the Guardian that a “massacre” by the Islamic Revolutionary Guard Corps was taking place. “They have shut down the city and are slaughtering people inside with guns and bombs just because they are chanting for freedom,” she said. Despite the authorities’ disruption of internet, videos showing apparent gunfire in Sanandaj have been posted online by the Norway-based human rights group Hengaw. Hengaw said Iranian war planes had arrived at the city’s airport overnight and buses carrying special forces were on their way to the city from elsewhere in Iran. On Monday – as protests sparked by the death of 22-year-old Mahsa Amini in police custody entered their fourth week – Britain said it was imposing sanctions against the “morality police in its entirety”, as well as against Iran’s police commander and the head of the Basij militia, linked to the Revolutionary Guards. The Iranian government responded by summoning the British ambassador to Iran, Simon Shercliff, to Tehran later the same day. Iran described the sanctions as “baseless” and accused the UK of interfering in its internal affairs. Protests have been especially intense in Sanandaj in Kurdistan, Amini’s home region, where rights groups fear heavy casualties. The New-York based Center for Human Rights in Iran said there was a risk of a similar situation in Sistan and Baluchistan province, in the south-east, where activists say more than 90 people have been killed since 30 September. “The ruthless killings of civilians by security forces in Kurdistan province, on the heels of the massacre in Sistan and Baluchistan province, are likely preludes to severe state violence to come,” its director, Hadi Ghaemi, told Agence France-Presse. In a new development on Monday, workers from Iran’s oil industry joined the demonstrations. Footage posted on Twitter showed workers blocking the road to the Bushehr petrochemical plant in Assaluyeh, on the Gulf, and chanting “death to the dictator”. A regional official said the workers were protesting over wages and not the death of Mahsa Amini. “The situation in Assaluyeh is really alarming,” the 23-year-old wife of an oil worker told the Guardian. “I am concerned about the safety of my husband. There is no way to communicate and reach him.” The woman, who said she had previously burned her hijab in protest over Amini’s death, added: “We will throw the regime out through our continued struggle this time.” Iran has the fourth largest reserves of crude oil in the world and the industry is key to its economy. Strikes of oil workers were a major factor in the success of the 1979 revolution. “If these unrests continue and expand, especially if the energy sector joins the protests, the regime will irreversibly be in trouble,” Fatemeh Aman, senior fellow at the Washington based Middle East Institute, said from Erbil. “I don’t know if at this point there is a will within the establishment to reconcile, but even if there is, bloody crackdowns on ethnic minorities [like in Sanandaj] will make any reconciliation almost certainly impossible.” The authorities have pinned the blame for the unrest and violence on a wide array of actors including armed Kurdish dissidents, American and Israeli agents, as well as “traitorous Iranians abroad”. No evidence of foreign involvement has been provided. France’s foreign affairs minister, Catherine Colonna, said five French citizens had been detained by Iranian authorities, after a video of a French couple confessing to “spying” was aired in Iran. A week prior, France urged its citizens to leave Iran saying they were at risk of arbitrary detentions. Prompted by the repressions of protesters, EU is set to join the US, Britain, and Canada in imposing sanctions of Iranian security forces. “The EU agreed yesterday the technical aspects of a sanctions package that will target those behind the repression,” Colonna said on Tuesday.