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 a counter-offensive in the spring.

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

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

"We're seeing a kind of a reduced tempo already of the conflict... and we expect that's likely to be what we see in the coming months," she said.

She said both Ukraine and Russian militaries would be looking to to prepare for any counter-offensive 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."

The move angered the United States and other Western nations with Washington accusing the group of siding with Russia.

Since October, oil prices have declined due to slower global growth and higher interest rates. Opec sources say the group is likely to approve a policy rollover.

Meanwhile, Ukraine's security service said the government in Kyiv is imposing sanctions against 10 senior Orthodox Church figures it accuses of having supported Russia's invasion.

The sanctions, which will freeze their assets for five years, are the latest moves by the Ukrainian government against religious groups seen as potentially subversive.