Nairobi - Rights groups on Wednesday urged the UN Security Council to impose "targeted sanctions" on Burundian officials over alleged human rights violations there, in an open letter published online.The letter, penned by Human Rights Watch and Reporters Without Borders (RSF) among others, was published on the eve of a meeting of the UN's top decision-making body to discuss the crisis in the Great Lakes nation."We... write to urge the UN Security Council to impose targeted sanctions against individuals responsible for ongoing serious human rights violations in Burundi," the 19 international and local rights group said. "Such measures, including travel bans and asset freezes, would send an important message to Burundian leaders who have faced little consequence for continuing to perpetrate gross abuses against their own people," the statement added.The United States and the European Union have already imposed sanctions on officials from the Burundian regime. Stalled by a Chinese and Russian veto, however, the UN Security Council has not followed suit.The Burundian authorities "have continued their broader policy of repression against suspected opponents, independent civil society, and media, and they have deliberately obstructed the ability of the UN to document rights violations", the NGOs wrote.They also accused the ruling party's youth militia of having "killed, tortured, raped, and severely beaten scores of people across the country". The UN's special adviser on preventing genocide, Adama Dieng, meanwhile warned of "mass atrocities" in Burundi and called on the UN Security Council to take "robust action" to quell the violence, in a letter to the body's members seen on Wednesday by AFP.Burundi's ambassador to the UN Albert Shingiro lashed out at Dieng's letter, branding it in a tweet "a theatrical show aimed at influencing the UN Security Council briefing".He also said his east African country was facing "diplomatic harassment".Hundreds of people have died in violence in Burundi triggered by President Pierre Nkurunziza's decision in April 2015 to stand in elections for a third term in office, which he went on to win.A September report by UN rights experts recounted spine-chilling cases of torture and horrific sexual violence, mass arrests and disappearances and warned that "the crime of genocide also looms large".More than 390,000 people have fled the country, and that figure is expected to surpass 500 000 this year, according to the UN refugee agency.