Loud explosions were heard over the Ukrainian capital of Kiev early Saturday as authorities warned of incoming ballistic missiles.

The air raid alert had just ended when explosions were heard throughout the city at 5:50 a.m. local time. Seconds later, air raid sirens began to wail.

The country’s air defense systems were working, the city’s mayor, Vitalij Klitschko, wrote on Telegram, warning of the missile threat and urging residents to stay in shelters.

Shortly after, he said emergency services were arriving at the scene of the fire. The regional military administration of the city announced that four people were killed.

Both sides in the nearly three-year war, which began with a full-scale Russian invasion of Ukraine in February 2022, have stepped up their attacks in in recent weeks.

This is a developing story.