Two Russian opposition parties agreed Friday to run on a joint platform in 2016 parliamentary elections, aiming to make a first step in uniting fractious Kremlin adversaries after the killing of party leader Boris Nemtsov.

Critics of President Vladimir Putin say he bears ultimate political responsibility for the gunning down of Nemtsov, a former deputy prime minister, on Feb. 27. The Kremlin denies stifling dissent and Putin has urged law enforcement bodies to investigate the case in full.

RPR-Parnas, co-founded by Nemtsov, and Party of Progress, led by anti-graft blogger Alexei Navalny, said they would also run together in local elections due in some regions this year.