The dead included Ukraine's interior minister, Denys Monastyrsky, his deputy, Yevhen Yenin, and state secretary, Yurii Lubkovych, Klymenko said.

Officials had earlier put the death toll at 18, but it was later revised down as authorities stressed the final figure was uncertain.

"The pain is unbearable," Ukrainian President Volodymyr Zelenskyy said of the death of the three children.

"May all those whose lives were taken this black morning rest in peace!” he said in a statement.

At least 30 people, including 12 children, were being treated in hospitals, officials said. The extent of their injuries was not immediately clear.

The helicopter fell near a kindergarten and a residential building, the head of the Kyiv Regional Military Administration Oleksii Kuleba said.

"At the time of the tragedy, children and employees of the institution were in the kindergarten. At this point, everyone was evacuated," Kuleba said.

Video and photos shared online by Ukrainian officials showed flames bursting from at least one building as smoke billowed up from the crash site.

The Security Service of Ukraine said on its Telegram channel that it was considering at least three possible causes for the crash, including malfunction, a violation of flight rules or “deliberate actions to destroy the vehicle.”

Ukrainian Prime Minister Denys Shmyhal said he had ordered a special commission to be created to lead an investigation into the circumstances of the incident.

Yurii Ihnat, spokesperson for Ukraine’s Air Force Command, said the commission would include a number of aviation experts. "It is too early to talk about the causes of the crash," Ihnat said, speaking during a national telethon.

A senior police official in the Kharkiv region said the helicopter was due to travel to the area in the country's northeast when it crashed.

There has been no fighting reported around Kyiv for months, since Russian forces pulled back from the region after a failed assault on the capital.

Officials in Ukraine and across Europe paid tribute to the dead.

“Tragedy far from the frontline, in Brovary,” foreign minister Dmytro Kuleba said on Twitter.

“I wish a speedy recovery to those injured. Denys Monastyrskyi and Yevhenii Yenin were close colleagues & friends of mine, true Ukrainian patriots. Huge loss for all of us,” he said.

Shmyhal, the prime minister, said it was “a great loss for the government team and the entire state."

"My sincere condolences to the families of all the victims," he said.

“My colleagues, my friends. What a tragic loss,” interior ministry adviser Anton Gerashchenko wrote in a tweet. “Deepest condolences to their families.”

Olena Zelenska, Ukraine's first lady, could also be seen dabbing away tears Wednesday morning before attending a World Economic Forum session in Davos, Switzerland, according to The Associated Press.

Forum President Borge Brende asked for 15 seconds of silence after opening the session to honor the Ukrainian officials killed in the crash.

The U.S. ambassador to Ukraine, Bridget A. Brink, said on Twitter that she was "shocked and saddened by the terrible news from Brovary."

Charles Michel, president of the European Council, remembered Monastyrsky as a "great friend of the EU."