However, there has been no evidence of fading resistance on the part of Ukrainian forces, US director of intelligence Avril Haines said.

She said both sides would try to "refit, resupply and reconstitute" for any counteroffensive in the spring.

It follows Russian attacks on Ukraine's critical energy infrastructure. 

The war in Ukraine is now in its ninth month but Russia has lost more than half the land it seized.

Haines told a defence forum in California that most of the fighting is currently around the Bakhmut and Donetsk regions of eastern Ukraine.

She said fighting had slowed down following Russia's withdrawal of troops from the west of the Kherson region last month.

"We're already seeing a kind of reduced tempo in the conflict... and we expect that to continue in the coming months."

She said both Ukraine and Russian militaries would be looking to prepare for any counteroffensive after the winter.

"But we actually have a fair amount of scepticism as to whether or not the Russians will be, in fact, prepared to do that," she said.

"I think more optimistically for the Ukrainians in that time frame."

Haines said US intelligence believes Russian President Vladimir Putin does not have a full picture at this stage of just how challenged his military is.

"We see shortages of ammunition, for morale, supply issues, logistics, a whole series of concerns that they're facing."

However, analysts at the Institute for the Study of War (ISW) painted a different picture in a daily review, questioning whether the freezing conditions could permit "a higher pace of operations as winter sets in".

Previously, the ISW asserted that operations in the Donbas were unlikely to slow down as freezing conditions created "the most optimal times of year for mechanised manoeuvre warfare".

BBC