A CIA agent was the "main projector" of deadly demonstrations in Iran with assistance from Israel and Saudi Arabia, the country's chief prosecutor alleged on Thursday.

Planning for the plot - dubbed "Consequential Convergence Doctrine" - was initiated four years ago by an operative from the Central Intelligence Agency, Iran's prosecutor Mohammad Jafar Montazeri was quoted by the state-run IRNA news agency as saying.

The CIA operative and an agent affiliated with Israel's Mossad intelligence agency were in charge of masterminding the unrest, while Saudi Arabia paid for all the expenses, he said.

The US government has denied having any hand in the protests and the CIA declined to comment on the allegations.

At least 22 people have been killed and more than 450 arrested since anti-government demonstrations erupted across Iran on December 28.

The protests, which have focused on economic and political grievances, have been the largest display of public dissent since pro-reform rallies swept the country in 2009.

Iran's interior minister said about 42,000 people took part in the week of demonstrations. Abdolreza Rahmani Fazli said in a statement Thursday the figure was "based on precise statistics we have".

"The relevant authorities have reported that there have been at most 42,000 people, and that is not a very high number," Fazli said.

The US Department of State said on Thursday it had the authority to hold the Iranian government responsible for the protester deaths.

"We condemn in the strongest possible terms the deaths to date and the arrests of at least 1,000 Iranians," Department of State spokeswoman Heather Nauert told reporters.

"We have ample authorities to hold accountable those who commit violence against protesters, contribute to censorship, or steal from the people of Iran. To the regime's victims, we say: You will not be forgotten," she said.

Iranian army chief General Abdolrahim Mousavi offered to assist police against anti-government demonstrators, though the protests have died down in recent days.

"In the event it is necessary, the army would take part in the fight against those led by the devil," Mousavi said on Thursday, according to the Asriran news portal.

Iran's Supreme Leader Ayatollah Ali Khamenei and other hardliners in the country's theocratic leadership have insisted for days the protests were fomented by foreign powers.

But moderate President Hassan Rouhani said the demonstrations were about jobs and human rights.