The US State Department cast doubt Monday on the integrity of Russia's parliamentary election, saying a government crackdown on critics had "prevented" citizens from exercising their civil rights.

"The September 17-19 Duma elections in the Russian Federation took place under conditions not conducive to free and fair proceedings," said State Department spokesperson Ned Price, adding that the crackdown "prevented the Russian people from exercising their civil and political rights."

He said the government had used laws against alleged extremist organizations, foreign agents, and undesirable organizations, a move that "severely restricted political pluralism" in the vote.

Those restrictions accompanied "widespread efforts to marginalize independent political figures," and also prevented independent foreign agencies from observing the elections, Price said in a statement.

"We call upon Russia to honor its international obligations to respect human rights and fundamental freedoms and to end its pressure campaign on civil society, the political opposition, and independent media."

The US statement came after Russia's opposition accused authorities of mass voter fraud after the election results Monday showed the ruling United Russia party, which supports President Vladimir Putin, winning a sweeping majority in parliament.