"The Secretary-General was shocked by and condemns the brutal killing of Boris Nemtsov in Moscow on 27 February," the United Nations said in a statement published on its official website on Saturday, adding that Ban Ki-moon is aware that an investigation into the murder has been announced and "expects the perpetrators to be brought to justice swiftly".
According to the statement, the UN Secretary General also "wishes to express his deepest condolences to Mr. Nemtsov's family, friends and supporters".
Russian President Vladimir Putin has ordered the Russian Investigative Committee, the Interior Ministry and the Federal Security Service to set up an investigative team to look into the murder of Boris Nemtsov.
Putin claimed that the murder was likely a contract killing.
The Russian Investigative Committee is at the moment considering several crime scenarios, with some of them linking the politician's murder to events in Ukraine, the Charlie Hebdo terrorist attack in Paris and Nemtsov's business activity.
Nemtsov has been in Russian politics for over two decades. He served as deputy prime minister and energy minister under the country's former President Boris Yeltsin. He co-chaired the RPR-PARNAS (Republican Party of Russia – People's Freedom Party) since 2012 and was also one of the leaders of the liberal Solidarnost movement.
Nemtsov voiced opposition to the current Russian government, President Vladimir Putin and his policies but was more actively involved in business in recent years.