US voices "serious concerns" about Russia vote

"We have serious concerns about the conduct of those ... elections," White House spokesman Jay Carney said at a news briefing.

Several thousand people protested in Moscow against the election, which has been condemned by European monitors.

Prime Minister Vladimir Putin's party lost some of its dominance in the 450-seat Duma, where it was set to see its majority decline to 238 seats from 315 in the parliament, according to early forecasts.

Secretary of State Hillary Clinton, who has sought to consolidate improvements in sometimes strained US ties with Moscow after a "reset" early in the Obama administration, said the United States was concerned by reports of stuffed ballot boxes, manipulated voter lists and other "troubling practices."

"Russian voters deserve a full investigation of all credible reports of electoral fraud and manipulation," Clinton said in Bonn, where she was attending an international conference on Afghanistan.

State Department spokesman Mark Toner said Moscow should fully investigate charges laid by monitors sent by the Organization for Security and Cooperation in Europe electoral watchdog branch.

He said the United States had spent more than $9 million on financial support and technical training for civil society groups before the election and would keep supporting those working to ensure free, fair and transparent electoral processes.

The Obama administration has repeatedly praised the results of the so-called reset in Russia ties, saying it had won progress on issues ranging from cutting nuclear arms to pressuring Teheran on its nuclear program.

But plans by Putin, known for his anti-US rhetoric, to return to the presidency have raised concerns and the two sides are at odds over US plans to set up an anti-ballistic missile shield to defend Europe against potential threats from Iran.