BRATISLAVA (Reuters) - The United States is concerned about Slovakia`s controversial new electoral law and has warned the government that it could hinder efforts to join NATO and the European Union, a senior U.S. official said. The Slovak parliament on Wednesday approved the new law, which the opposition described as undemocratic and framed to ensure victory for Prime Minister Vladimír Mečiar`s government at September`s general election. Slovakia has been criticised on several occasions by the United States for backsliding on democracy. Stephen Flanagan, a special adviser to President Bill Clinton, told a news conference in Bratislava he had asked Slovak representatives, including parliament chairman Ivan Gasparovic, to revise the legislation before it becomes valid. "Our government`s concerns about the electoral law remain," Flanagan said. "Whether the elections will be free and fair will have no small influence on (Slovakia`s) integration into the European and transatlantic structures," he added. Slovakia was not invited among the first wave of applicants to begin talks on EU and NATO membership with both organisations citing concerns about democracy. The law, proposed by deputies of Meciar`s Movement for a Democratic Slovakia (HZDS), changes the rules just four months before the election and two months before the deadline for the submission of party lists. "We want to see free and fair elections in Slovakia in September," Flanagan said. "But we emphasise that it`s not only what happens on election day that counts, but it`s what happens during the entire process." The new law requires each party within a coalition to cross a five-percent threshold of the overall vote to gain representation in parliament. This would most seriously affect the leading opposition grouping, the Slovak Democratic Coalition. It also bans election campaigning in private media. Flanagan said the United States was concerned that this aspect of the law was too vague and did not provide a clear definition of what the word "campaigning" actually meant. Meciar frequently accuses Slovakia`s privately-owned media of bias against the government and says the new law brings Slovakia into line with international norms.