Unconfirmed reports said that Russian forces have destroyed or rendered unusable the Ukrainian Navy, and taken control of Boryspil Airport in Kiev, the Daily Mail reported.
Access to the Black Sea and Azov Sea was cut off, while more unconfirmed reports on Twitter appeared to show a huge seaborne landing by Russian forces in the Black Sea port of Odessa, involving large landing craft and helicopters shortly before 6 a.m. on THursday morning, the report said.
A government spokesman also said that the country was attacked from Crimea, which Russia had annexed from Ukraine in 2014.
"The work of enemy sabotage and reconnaissance groups is also recorded. Depending on the situation on the border, border guards together with the Armed Forces of Ukraine and the National Guard of Ukraine are firing at the enemy.
"Information on injuries among border guards is being clarified," the Daily Mail quoted the spokesman as saying.
Putin said in a televised address on Thursday morning that in response to a request from the heads of the Donbas Republics of Donetsk and Luhansk, he had decided to conduct a special military operation to protect the people "who have been suffering from abuse and genocide by the Ukrainian regime for eight years".
He said Russia could not exist with a "constant threat emanating from the territory of Ukraine" and clashes between Russian and Ukrainian solders was "inevitable", Daily Mail reported.
Ukraine's President, Volodymyr Zelenskyy declared martial law in the early hours of Thursday, in a video message to the people urging people to remain at home and stay strong.
He said he had just spoken to US President Joe Biden.
"We are working. The army is working. Don't panic. We are strong. We are ready for everything. We will defeat everyone."