U.S. urges Russia to probe vote-fraud charges Updated at 1:13 p.m. (AP) MOSCOW - The Obama administration on Monday congratulated the Russian people for turning out to vote in big numbers in this weekend's presidential election but said it was concerned by allegations of massive fraud and pointedly did not mention victor Vladimir Putin by name. In a statement, the State Department said the U.S. would work with Russia's "president-elect" once the vote is certified. It noted that European observers had determined that "the election had a clear winner with an absolute majority." At the same time, it pointed out that the monitors had raised issues with the fairness of the campaign, partisan use of government resources and procedural irregularities on election day. It said those charges must be fully and credibly investigated. Thousands of Russians challenge Putin's election Observers: "Serious problems" in Russia vote Putin's victory just "honest" enough? "The United States congratulates the Russian people on the completion of the presidential elections, and looks forward to working with the president-elect after the results are certified and he is sworn in," State Department spokeswoman Victoria Nuland said in the statement. She also welcomed some improvements in the electoral process but said additional steps would be needed to make it more transparent. Putin won more than 63 percent of the vote according to the nearly complete official returns from Sunday's election. But the opposition and independent observers say the election has been marred by massive fraud and thousands of Russians gathered Monday for a massive rally to challenge Putin's victory, chanting "Shame!" and "Russia without Putin!" The demonstrators are contesting the outcome of the vote, pointing to a campaign heavily slanted in Putin's favor and to reports of widespread violations in Sunday's ballot. "We urge the Russian Government to conduct an independent, credible investigation of all reported electoral violations," Nuland said. According to the independent Russian elections monitor Golos, many voters cast their ballots either right at their state-run enterprises or under their bosses' watchful eyes, while others were bused to polling stations where they were not on voter rolls and where there was little oversight over voting. Not free or fair Andrei Buzin, an election expert at Golos, said that the falsifications were not widespread enough to have left Putin with less than 50 percent of the vote and require a runoff, but the vote was still skewed. "I wouldn't call these elections free or fair," Buzin said. International observers The Organization of Security and Cooperation in Europe said "there was no real competition, and abuse of government resources ensured that the ultimate winner of the election was never in doubt." The watchdog noted that the election "process deteriorated during the vote count, which was assessed negatively in almost one-third of polling stations observed."