Russian forces in Ukraine have reportedly started a new major offensive in the Luhansk region, in a bid to take full control of the country's two eastern regions, Luhansk and Donetsk, together known as Donbas.

Russian forces have for some time kept up fierce attacks in the Donetsk region, as they try to encircle Bakhmut, a major operational base of the Ukrainian forces.

The Institute for the Study of War, a US think tank, said in its analysis posted on Wednesday, "Russian forces have regained the initiative in Ukraine and have begun their next major offensive in Luhansk."

The institute went on to say that the pace of Russian operations along the Svatove-Kreminna line in western Luhansk has increased markedly over the past week.

It added that Russian forces are deploying elements of a motor rifle division, a tank division, and an airborne division, in conducting offensive campaigns there.

The institute said, "Ukrainian forces are so far preventing Russian forces from securing significant gains." But it also said that the Russian offensive has likely not yet reached its full vigor.

Ukraine's national security chief, Oleksiy Danilov, said on Tuesday that Kyiv is on high alert in anticipation of the start of a major Russian offensive before the one-year anniversary of its invasion on February 24.

In an interview with Reuters, Danilov, who heads the National Security and Defense Council, said that Kyiv expects Russia to include the northeastern Kharkiv or southern Zaporizhzhia regions as targets of the anticipated offensive.

He said the Russians "need to have something to show before their people, and have a major desire to do something big." He said, "How successful they will be will depend on us."