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

An air-raid alert had just ended when booms 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, Vitali Klitschko, wrote on Telegram, warning of the missile threat and urging residents to stay in shelters.

Shortly after, he said that emergency services were responding to the scene of a fire. The city’s regional military administration said that four people had been killed.

Both sides in the nearly three-year war, which began with Russia’s full-scale invasion of Ukraine in February 2022, have ramped up their assaults in recent weeks.

This is a developing story.