U.N. officials say a 2015 agreement to bring peace to Ukraine's volatile east remains largely unimplemented and civilians are paying the highest price, with more than 3,300 killed and 3.5 million needing humanitarian aid this year.

Russia's annexation of the Crimean Peninsula in early 2014 and support for separatist rebels in the east triggered a conflict with Ukrainian government forces that the U.N. says has also injured up to 9,000 civilians and displaced 1.5 million people.

Assistant Secretary-General Miroslav Jenca told the Security Council on Tuesday that negotiations "appear to have lost momentum," with Russia and Ukraine unable or unwilling to agree on key steps forward or too distracted to focus on implementing the 2015 agreement.

Russia and Ukraine blamed each other for the failure to implement the agreement.