Meanwhile, Ukrainians did confirm that Russia has pulled back from Chernobyl.
Russia threatened Thursday to turn off its gas taps to Europe, opening up a new front in its war in Ukraine amid growing scepticism over Moscow's claim it is scaling back its onslaught.
Over a month into Russia's invasion of its neighbour, Vladimir Putin's troops have devastated cities like Mariupol with shelling, killing at least 5,000 people in the port city alone.
But they have struggled to take any significant territory.
Moscow insisted things were going to plan as it said this week it would scale back attacks on capital Kyiv and the northern city of Chernigiv.
But Ukrainian President Volodymyr Zelensky has dismissed the promise as a red herring, and US President Joe Biden said he was "sceptical."
Instead, the US and NATO shared Zelensky's reading that Moscow may be seeking to regroup and resupply for its offensive in the eastern Donbas region.
Russia has moved a "small number" -- perhaps 20 percent -- of its troops from around Kyiv after failing to capture the city, which continues to be targeted by Russian airstrikes, Pentagon spokesman John Kirby said.
"It's not exactly clear where they're going to go, for how long, and for what purpose," he said, "but we don't see any indication that they're going to be sent home."
He said the "best assessment" is that the troops are "going to be repositioned, probably into Belarus, to be refit and resupplied and used elsewhere in Ukraine."
He noted that Russia has said it plans to "reprioritize" its operations in the Donbas area.
Underscoring Russia's underestimation of Ukraine's dogged defence, Biden said there was some indication that Putin was out of touch with the situation on the ground.
Western intelligence has warned the Russian leader's advisers may be "afraid to tell him the truth" about battlefield losses or the calamitous damage that sanctions have wrought on the country's economy.
But Kremlin spokesman Dmitry Peskov rejected the assessments, saying Western officials "don't understand President Putin, they don't understand the mechanism for taking decisions and they don't understand the style of our work".
Biden was notably cautious.
Putin "seems to be self-isolated and there's some indication that he has fired or put under house arrest some of his advisers," the US president told reporters.
But he said he did not want to put "too much stock" into the reports.
On the ground, shells continued to rain down on Kyiv and the northern city of Chernigiv, where the governor of the region, Vicheslav Chaus, poured scorn on Moscow's claim it was deescalating.
"At the minimum, it is regrouping," he wrote on Telegram.
The Ukrainian commander in the eastern city of Kharkiv also warned Russian forces were "regrouping to attack and put maximum forces in the region."
General Pavlo "Maestro" told AFP his message to the Russian invading force was: "go home while you're still alive" -- though he warned against underestimating Moscow.
The Ukrainians did confirm that Russia has pulled back from one area: Chernobyl.
"There are no longer any outsiders on the territory of the Chernobyl nuclear power plant," officials in Kyiv said, after weeks of occupation.
- US taps oil stockpile -
With his economy crippled by unprecedented international sanctions, Putin on Thursday sought to leverage Russia's status as an energy power, warning that EU members will need to set up ruble accounts from Friday to pay for Russian gas.
The EU has joined the United States in imposing sanctions -- however, mindful of their own power needs, the bloc has stopped short of a full-on energy embargo.
"If such payments are not made, we will consider this a breach of obligations on the part of our buyers" and existing contracts would be stopped, Putin said.
Germany, which before Putin's offensive in Ukraine imported 55 percent of its gas supplies from Russia, insisted that it will pay in euros or dollars as stipulated in the contracts.
Berlin and Paris were also "preparing" for a scenario where Russian gas simply stops flowing, France's economy minister said.
Biden moved to mitigate the damage to the overheated oil market by announcing a release from strategic US reserves of a million barrels daily for six months.
It is by far the largest tapping of the stockpiles in US history, and amounts to augmenting global supplies by about one percent.
- Pivot to Donbas? -
Military experts believe that with thousands of Russian troops killed and many thousands more injured amid unexpectedly fierce Ukrainian resistance, Moscow has to ditch efforts to advance simultaneously in the north, east and south.
Its focus instead has turned towards capturing more urban centres in the Donbas area including the besieged port city of Mariupol, while continuing to fire long-range assaults on other cities.
Russia forces have encircled Mariupol, which the Kremlin wants to capture to ensure an unbroken link between the cities of Donetsk and Lugansk, under de facto Russian control, and the Crimean peninsula.
But a larger push into the Donbas region could herald a more prolonged conflict, Pentagon spokesman John Kirby said Thursday.
"It might not just be a matter of days and weeks, it could be much longer than that," he said.
In Mariupol, where tens of thousands have for weeks been under siege with little water, food or electricity, Ukrainian authorities sent 45 buses in for a new rescue mission.
The international Red Cross said it was "ready to lead the safe passage operation" on Friday. Previous repeated attempts to get civilians out have collapsed.
Zelensky warned his war-torn nation to brace in particular for a new Russian onslaught in the Donbas.
"There is an accumulation of Russian troops for new strikes in Donbas and we are preparing for it," he said in a video message late Wednesday. "We will fight for every metre of our land."
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)