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.

The head of the Security Service of Ukraine, Vasyl Malyuk, said a day after the strike that 41 enemy aircraft were destroyed. Other assessments, however, including one by BBC News Ukraine, tallied a more modest number, upward of 11 destroyed jets.

President Volodymyr Zelenskyy said Defense Minister Rustem Umerov would sit down with Russian officials at the second round of peace talks on Monday.