People in Ukraine describe the moment they awoke in a war zone as Russian forces bombed the cities where they live

• Russian forces invaded Ukraine on Thursday morning, launching airstrikes on Kyiv and other cities.
• People across Ukraine awoke to the sounds of sirens and explosions, eyewitnesses told Insider.

Russian forces invaded Ukraine by land, air, and sea on Thursday morning, in what NATO Secretary-General Jens Stoltenberg condemned as a "brutal act of war."

People across Ukraine were jolted from their sleep by the sounds of sirens and explosions as Russian airstrikes hit the country's capital and other cities.

Eyewitnesses told Insider that this was their first realization that a full-scale invasion was underway.

In Kyiv, Ukraine's capital, the airstrikes started before dawn, and across the city of 3 million people, the sound of explosions, gunfire, and sirens was heard, Reuters reported.

"I woke up at five in the morning to the sounds of the blasts. We all did," said Ivanna Klympush-Tsintsadze, a member of the Ukrainian parliament with the European Solidarity Party, speaking to Insider from her home in Kyiv.

Bryan Stern from Project Dynamo, a group helping to evacuate American citizens from Ukraine, said his "entire room shook" when a missile landed near his hotel room in downtown Kyiv. "It was pretty intense," he added.

Witnesses also described missile blasts in other cities, including Kramatorsk, Dnipro, and Odesa, reports said.

Minutes after President Vladimir Putin announced a military operation on Thursday morning, The New York Times reported, explosions were visible in Kharkiv, Ukraine's second-largest city.

Makama Ezekial, a South African living in Kharkiv, said the "crashing sounds" of missiles exploding made him jump out of bed. Ezekial said he immediately went to find his children because he was afraid for their safety.

His children were fine. But Reuters reported that emergency services said a boy was killed in Kharkiv after shelling struck an apartment complex.

In Odesa, a picturesque tourist hub in the southwest of Ukraine, a local journalist said he heard "roughly 20 rockets" explode near his home on Thursday morning.

"Then we saw the smoke from the balcony," said Ugo Poletti, the editor in chief of The Odessa Journal.

Regional authorities of Ukraine's Odesa region said 18 people were killed in a missile attack, Reuters reported.

But despite the "scary scenario," Poletti said the citizens of Odesa appeared to be "quite calm."

Yosri Lahouar, a Tunisian living in Ukraine, was also struck by the calmness of the locals in Odesa.

A missile landed about 10 meters away from Lahouar's apartment, he said. He immediately grabbed his backpack and ran out to the streets to find shelter as smoke engulfed him, he said.

Then 10 minutes later, he continued, there was another explosion nearby. "I was on the ground, and I started to cry," he said.

Lahouar said his reaction stood out from the crowd. "When I looked around, everyone was walking around normally, like nothing had happened," he said. "I was surprised."

The explosions, Lahouar added, were a moment of clarity for him. He said, "At first, I was one of the voices who thought that war would never happen." But as of this morning, he said, he had to come to terms with the fact that he was living in a "war zone."