Russian President Vladimir Putin on Wednesday introduced martial law in Ukraine's Donetsk, Lugansk, Kherson and Zaporizhzhia regions that Moscow claims to have annexed.

"I signed a decree to introduce martial law in these four subjects of the Russian Federation," Putin said during a televised National Security Council meeting.

The Kremlin then published a decree saying martial law will be introduced from early Thursday.

Putin's announcement came as Ukrainian forces advanced in territory held by Moscow for months.

"The Kyiv regime refused to recognize the will of the people, rejects any proposals for negotiation, gunfire continues, civilians are dying," the 70-year-old Russian leader said.

He accused Ukraine of using "terrorist methods".

"They send sabotage groups into our territory," he said, claiming Moscow had foiled other attacks after its Crimea bridge was targeted "including at our nuclear power facilities."

Under Russian law, martial law allows for the strengthening of the military, curfews, limits of movement, censorships and interning of foreign citizens.

"We are working on solving very complex large-scale tasks to ensure security and protect the future of Russia," Putin said.