WASHINGTON—Ukraine’s weekend attacks against military airfields deep inside Russia signal the long-stalemated war is entering a perilous phase, with both sides seemingly intent on escalation and prospects for a U.

Officials from Russia and Ukraine in Istanbul on Monday failed to reach a ceasefire but agreed to exchange more prisoners.

It restated Moscow's demands that Ukraine become a neutral country - ruling out membership of NATO - and that it protect the rights of Russian speakers, make Russian an official language and enact a legal ban on glorification of Nazism. Ukraine rejects the Nazi charge as absurd and denies discriminating against Russian speakers.