Russian President Vladimir Putin says he has signed a decree to introduce martial law to the four regions annexed by his country from Ukraine.

Putin announced the measure on the regions of Donetsk, Luhansk, Kherson and Zaporizhzhia at an online meeting of the Russian Security Council on Wednesday.

He criticized the Ukrainian administration, saying it has refused negotiations and continues to shell Russian territories.

Putin has defined his country's invasion of Ukraine as a "special military operation."

With the latest measure, he apparently made clear his intention to place the four Russian-occupied regions on a war footing as Ukraine's military has stepped up counteroffensives.