A US think tank says Russian forces are preparing for possible Ukrainian advances on the east bank of the Dnipro River in southern Ukraine's Kherson region.

Russian forces withdrew from the western side of the river in November.

The Institute for the Study of War released its latest assessment on Sunday. The report says Russian forces are building large-scale defensive positions on the eastern side of the river.

It adds Russia has likely deployed mobilized personnel to the primary lines of defense.

The US think tank also said the Russian military has clearly assessed that Ukrainian forces could cross the Dnipro River and conduct counteroffensive operations in the eastern Kherson region, possibly threatening critical ground lines of communications from Crimea to Russia.

Moscow unilaterally annexed Crimea eight years ago.

The institute says, "The Russian effort to prepare extensive defensive positions in eastern Kherson, however well or poorly executed, highlights the critical importance this terrain holds for the future course of the war."

Meanwhile, Ukrainians continue to be affected by severe power shortages after Russia attacked energy facilities in the country.

The state-run utility Ukrenergo on Monday said it had implemented emergency power cuts across the country.

Ukraine officials are racing to set up special tents where people can shelter in the event of major blackouts.

In the capital Kyiv, officials say more than 400 tents had been set up across the city as of Monday.

NATO's head criticized Russian President Vladimir Putin at a news conference on Monday for disrupting energy supplies in Ukraine.

Secretary-General Jens Stoltenberg said, "Russia will likely continue attacking Ukraine's power grid, its gas infrastructure and basic services for the people."

He said, "Doing that when we enter winter demonstrates that President Putin is now trying to use the winter as a weapon of war against Ukraine."