A senior Ukrainian government official has reportedly said his country is preparing for a major escalation by Russian forces and not excluding any scenario in the next two to three weeks.

National Security and Defense Council Secretary Oleksiy Danilov was speaking to the UK's Sky News on Tuesday. He referred to the possibility that Russian forces will attempt a major offensive this month to coincide with the anniversary of its February 24 invasion.

Danilov said Ukraine's military is preparing for all possibilities. He said the hardest fighting is yet to come and the next few months will be crucial in deciding the course of the war.

The Institute for the Study of War, a US think tank, says in its assessment released on Tuesday that Russia has introduced forces to the frontline in Bakhmut, a Ukrainian stronghold in the eastern region of Donetsk, and retained the initiative for its operations.

The institute says Russian conventional forces may be replacing those of the private military company Wagner Group.

Ukraine's military has been urging Western countries to send more weapons, including fighter jets, in addition to tanks, apparently to prepare for a major counteroffensive said to be planned for spring or later.