(CNN) -- The United States is "disappointed" by the refusal of Russia's Ministry of Justice to register the liberal Party of People's Freedom on Wednesday, effectively barring it from participating in upcoming elections for the Russian assembly, or Duma, Secretary of State Hillary Clinton said in a statement.
"We are troubled by reports of pressure from authorities in the regions designed to intimidate PARNAS (the party's) supporters, prompting them to resign positions or disavow their signatures on required lists," Clinton said.
"The right to hold free, fair, competitive elections is a universal principle that the Russian government has repeatedly endorsed. It is hard to understand how this decision today by the Ministry of Justice is consistent with Russia's international commitments and recent statements by Russia's own leaders," Clinton said.
She urged an investigation into reports of irregularities in the party's registration process "to ensure that the procedures used to deny registration of this party were consistent with Russian laws and international standards."
The party was founded by Russian opposition leaders, including co-chairman Mikhail Kasyanov, a former prime minister. Kasyanov told the Russian news agency Interfax on Wednesday he believes the decision not to register the party came from top Russian officials, not the Ministry of Justice.
"It is clear that (Prime Minister Vladimir) Putin has decided not to admit our party to the elections," Kasyanov told Interfax. "PARNAS' participation in the elections would have involved serious risks for Putin's vertical power structure."
He said Russian officials were obligated to register the party, which has 46,000 members.
The Russian government had no immediate comment on the matter.
"We haven't seen the final report from the Ministry of Justice about their decision," State Department spokesman Mark Toner told reporters Wednesday. "But just speaking broadly about the refused registration, we believe that a lot of the irregularities that they cited were minor. There were a very small number of invalid signatures and other minor administrative errors.
"And again, we're concerned by reports that we've seen that some of the individuals were pressured to disavow their support for PARNAS. So I think what we're seeing is ... a concerning trend, if you will," Toner said. "And we're calling attention to it."
U.S. ties with Russia have improved recently, thanks to a positive relationship between the Obama administration and Russian President Dmitry Medvedev and Russian cooperation on issues such as Afghanistan.
Last month, however, Medvedev threatened that Russia would need to develop a "significant nuclear potential" if the United States does not convince Moscow its Europe-based missile defense system isn't aimed at Russia.
But the two nations have signed the Strategic Arms Reduction Treaty (START), which expired in December 2009, and the New START, designed to replace it. The treaty is designed to slash the stockpile of strategic nuclear weapons in both countries to 1,550 warheads -- down from a current cap of 2,200 -- and 700 launchers.
The 2010 U.S. State Department Human Rights Report on Russia said, "There were numerous reports of governmental and societal human rights problems and abuses during the year. The restrictions on political competition and interference in local and regional elections in ways that restricted citizens' right to change their government continued."
In January, Medvedev told business and political leaders in Davos, Switzerland, that his nation is trying to modernize but recognizes it has work to do.
"Russia is very often criticized. Sometimes the criticism is well-deserved, sometimes absolutely not," he said in a speech at the World Economic Forum. "Russia is rebuked for the lack of democracy, totalitarian tendencies, weaknesses of legal and judiciary systems."
"We are learning and we are willing to receive friendly advice," Medvedev said. "But what we don't need is lecturing -- we should be working together."