KYIV, Ukraine—Belarusian authorities blocked the website of a leading online media outlet, detained several of its journalists, and conducted searches involving three other media organizations Thursday, the latest moves in a sweeping clampdown on dissent and independent media in the ex-Soviet nation. Belarus’s Information Ministry said it has blocked Nasha Niva’s website after the prosecutor general’s office had accused it of posting unspecified unlawful information. The Belarusian Association of Journalists, or BAJ, said that the authorities conducted searches at Nasha Niva’s offices, detained chief editor Yahor Martsinovich and editor Andrey Skurko, and searched their apartments. Another four Nasha Niva journalists couldn’t be reached, the BAJ said. Agents of the Belarusian state security agency, which still goes under its Soviet-era name, KGB, also conducted searches Thursday at two regional media outlets, the Brest Gazette in the city of Brest on the border with Poland and the Intex-press in the city of …