Explosions were reported over the Ukrainian capital Kyiv in the early hours of Thursday.
Both the Reuters and AFP news agency cited people on the ground as saying they heard explosions.
Russia has struck Kyiv with ballistic missiles, said the city's mayor, Vitaly Klitschko, urging people to seek shelter.
It comes as Ukrainian President Volodymyr Zelenskyy had warned of a "high probability that the strike will be carried out tonight."
"I received a report from Air Force Commander Anatolii Kryvonozhko. The Russians prepared a massive attack several days ago," Zelenskyy said on X.
