VILNIUS, Sept 7 (Reuters) - The Belarus government"s crackdown on human rights organizations obstructed the ability of the United Nations to document abuses, the U.N. envoy said on Tuesday. Belarus was levied with several rounds of international sanctions after a crackdown over mass protests against President Alexander Lukashenko, whose opponents say he fraudulently won last year"s presidential election. He denies wrongdoing. In the latest moves against Lukashenko"s opponents, Belarusian security police searched offices and homes of lawyers and human rights activists on July 14, detaining at least 10 people including the leader of Viasna-96 human rights group. read more The curbs on the largest human rights body in the former Soviet country are hindering international monitoring of human rights abuses in Belarus, U.N. Special Rapporteur Anais Marin told reporters in Vilnius, capital of Lithuania. She is not allowed into the country, and she said contributions from Viasna and its volunteers have been "crucial" in her mission. "The terrible repression that is currently targeting Viasna has a direct negative impact on our capacity in Geneva to follow up on allegations of human rights violations, to gather testimonies and to properly report on the repression, and this is probably no coincidence", she said. Speaking by a videolink, the rapporteur called the detention of Viasna"s leadership "arbitrary" and said their prosecution was "politically motivated." She called for their release and urged that "they would not be subjected to ill-treatment in detention". "Last year, I assessed the situation in Belarus as catastrophic. I"m lacking words now to express my interpretation of the situation. I could not imagine it could get so much worse", Marin said. On Monday, Maria Kolesnikova, one of the leaders of the mass street protests last year, was sentenced in Belarus to 11 years in prison, leading to an outcry from Western countries.