Fighting has raged in Ukraine since Russia launched a full-scale invasion more than three years ago. Russian forces have slowly expanded the amount of territory they control over the past year, mostly in the east of Ukraine.

Ukraine and Russia stepped up strikes ahead of Moscow’s World War II commemorations on Thursday, with Russia closing more than a dozen airports and canceling scores of flights amid a wave of drone attacks.

The attacks came a day before a Russian-proposed cease-fire was to take effect and as Moscow prepared to welcome foreign dignitaries for a parade.