Iran's notorious Evin prison and the paramilitary group Ansar-e Hezbollah have been hit with new US sanctions, for allegedly committing "serious human rights abuses" against its political dissidents and critics of the government.

In an announcement late on Wednesday, US Treasury Secretary Steven T Mnuchin said the two entities, as well as six individuals and a communications technology agency, played a role in the "brutal crackdown" of demonstrators following the recent deadly protests in the country.

Iran rejected the accusations of human rights abuses as "untrue" and "politically motivated", the official news agency PressTV said. 

Located in northern part of capital Tehran, Evin prison is notable for keeping political prisoners, dissidents and dual Iranian nationals accused of plotting against the government.

In February, Seyed Emami, a top environmentalist and university professor, died of mysterious circumstances while being held at Evin. British Iranian national Nazanin Zaghari-Ratcliffe is also being held at Evin after being convicted in 2016 of conspiring to topple the Iranian leadership.

Evin's prison guards have also been accused of torturing and killing people in custody. In 2003, a Canadian Iranian journalist, Zahra Kazemi, died of "brain haemorrhage resulting from beatings" after being detained for taking pictures of the prison.

The US alleges that Iran's intelligence ministry and the Revolutionary Guard maintain permanent wards at Evin, "where they hold political prisoners".

"Those who speak out against the regime's mismanagement and corruption are subject to abuse and mistreatment in Iran's prisons," Treasury Secretary Mnuchin said.

The paramilitary group Ansar-e Hezbollah is also being targeted with sanctions for its alleged involvement "in the violent suppression of Iranian citizens" and for working with the Basij, a special Revolutionary Guard unit, to carry out attacks on student protesters using "knives, tear gas and electric batons".

Three officials of the group were also included in the list including Hamid Ostad, a local leader in Mashhad, for being allegedly involved with a "mob attack" of Saudi Arabia's consulate in that holy city.

Agencies