Ukrainian military command centers in the cities of Kyiv and Kharkiv have been attacked by missile strikes, the Ukrainska Pravda news website cited a Ukrainian interior ministry official as saying on Thursday, February 24.

Reuters witnesses in Kyiv heard a series of explosions shortly after Russia announced a military operation in Ukraine.

In a pre-dawn TV statement Russian President Vladimir Putin said Russia did not plan to occupy Ukraine, but said Moscow's response would be "instant" if anyone tried to stop this.

Ukraine said that "Putin has launched a full-scale invasion of Ukraine".

In a video statement on Thursday morning, Ukrainian President Volodymyr Zelensky said martial law was now being imposed across all of Ukraine, the BBC reports.

"No panic. We're strong. We're ready for anything. We'll defeat everyone, because we are Ukraine," the president said.

He said Russia carried out strikes on Ukraine's military infrastructure and border guard units. Military headquarters, airfields and military warehouses in Kyiv, Dnipro and Kharkiv, officials said.