Iran's foreign ministry Monday called on Washington to "stop violence" against its own people after protests across the US over the death of a black American man.

"To the American people: the world has heard your outcry over the state of oppression. The world is standing with you," foreign ministry spokesman Abbas Mousavi said at a news conference in Tehran. "And to the American officials and police: stop violence against your people and let them breathe," he told reporters in English.

Tens of thousands of protesters have taken to the streets across America demanding tougher, first-degree murder charges and more arrests over the death of George Floyd at the hands of a Minneapolis police officer.

The unarmed black man stopped breathing after being handcuffed and as a white police officer, Derek Chauvin, knelt on his neck for nearly nine minutes. Chauvin has been fired and was charged with third-degree murder on Friday, five days after Floyd's death. "We deeply regret to see the American people, who peacefully seek respect and no more violence, being suppressed indiscriminately and met with utmost violence," Mousavi told reporters in English.

He also accused Iran's sworn enemy the United States of "practising violence and bullying at home and abroad".

The violent protests in the US have received widespread coverage in Iranian media, especially on state television, which recently aired a programme accusing America of institutional racism. Tensions between Tehran and Washington have been rising since 2018, when President Doland Trump withdrew the US from a landmark nuclear accord and reimposed crippling sanctions on Iran's economy.

Iran was condemned by the United States after deadly street violence that broke out in the Islamic republic in November during protests against a surprise petrol price hike. The United States said afterwards that more than 1,000 people may have been killed in the Iranian crackdown.

London-based human rights group Amnesty International has said 304 people were killed in that crackdown, including 12 children. Officials in Iran are yet to issue an overall death toll for the unrest, however.

They have repeatedly denied death tolls given by foreign media and human rights groups as "lies", and passed responsibility for reporting on it between various state bodies. But Iran's Interior Minister Abdolreza Rahmani Fazli suggested that up to 225 people were killed, in a report by ISNA news agency on Sunday.

A group of independent UN rights experts said in December that based on unconfirmed reports more than 400 people could have been killed in the crackdown.