Explosions have been heard in Kyiv and other Ukrainian cities, a day after Russia accused Ukraine of carrying out a drone attack on the Kremlin.
Attacks were also reported in Zaporizhzhia and Odesa in the south.
The air strikes come as Ukrainian President Volodymyr Zelensky is due to speak at The Hague as part of an unexpected visit to the Netherlands.
He will also visit the International Criminal Court, which is investigating alleged Russian war crimes in Ukraine.
Russia has accused Ukraine of attempting to assassinate President Vladimir Putin, but Mr Zelensky denied that his country carried out the attack.
On Wednesday, he said: “We don’t attack Putin or Moscow. We fight on our territory. We are defending our villages and cities.”
Mr Zelensky was speaking in Finland, where he made a surprise visit and met his Finnish counterpart Sauli Niinisto and the leaders of Sweden, Norway, Denmark and Iceland.
During his visit to the Netherlands, Mr Zelensky is expected to meet Dutch Prime Minister Mark Rutte. Dutch media said that the two were likely to discuss Mr Zelensky’s demands for more military support – namely long-range weapons and fighter jets.
Last January, Mr Rutte said that supplying fighter jets was not taboo, although such a move would be “a really big next step”.
Following the alleged drone attacks, Russia threatened to retaliate when and where it considered necessary.
On Wednesday, Russian strikes on Ukraine’s southern Kherson region killed 21 people. Officials said that the victims included supermarket customers and employees of an energy company who were performing repairs.
And in the early hours of Thursday morning, air raid sirens rang out across many Ukrainian regions. Loud blasts were reported in Kyiv and Odesa.
At the same time, a drone hit an oil refinery in southern Russia, setting part of it on fire – the latest in a series of explosions, fires and drone attacks that have occurred in Russia in recent weeks.
Some commentators have argued that the alleged drone strike on the Kremlin was internally conducted and purposefully staged by Russia.
The Institute for the Study of War said it is “extremely unlikely that two drones could have penetrated multiple layers of air defence and detonated or been shot down just over the heart of the Kremlin in a way that provided spectacular imagery caught nicely on camera”.
It said that “Russia likely staged this attack in an attempt to bring the war home to a Russian domestic audience and set conditions for a wider societal mobilisation”.
Yurii Ihnat, a spokesman for the Ukrainian Air Force, said he thought Russia had staged the attack on the Kremlin to try to “show some kind of escalation on the part of Ukraine”.
But other commentators disagreed, saying that Russia would have little interest in making itself look “weak” by staging an attack that makes the Kremlin look vulnerable.
It would also lead to questions about how well-protected Mr Putin is – and about the effectiveness of Russian air defences.