The official spoke on condition of anonymity under ground rules set by NATO.
But with Wednesday marking four full weeks of fighting, Russia is bogged down in a grinding military campaign, with untold numbers of dead, no immediate end in sight, and its economy crippled by Western sanctions.
US President Joe Biden and key allies are meeting in Brussels and Warsaw this week to discuss possible new punitive measures and more military aid to Ukraine.
As Mr Biden left the White House on Wednesday for the flight to Europe, he warned there is a “real threat” Russia could use chemical weapons and said he will discuss that danger with the other leaders.
In an apparent reflection of growing divisions in Russia’s top echelons, top official Anatoly Chubais resigned, Kremlin spokesman Dmitry Peskov told the Interfax news agency.
Mr Chubais, the architect of Russia’s post-Soviet privatisation campaign, had served at a variety of top official jobs over three decades. His latest role was as Mr Putin’s envoy to international organisations.
Mr Peskov would not say if Mr Chubais had left the country.
With his olive-drab T-shirts, unshaven face and impassioned appeals to governments around the world, Ukrainian President Volodymyr Zelenskyy has been transformed into a wartime leader and Russian President Vladimir Putin’s number one antagonist.
Addressing Japan’s parliament on Wednesday, Mr Zelenskyy said four weeks of war had killed thousands, including at least 121 of Ukraine’s children.
“Our people cannot even adequately bury their murdered relatives, friends and neighbours,” he said.
“They have to be buried right in the yards of destroyed buildings, next to the roads.”
Repeatedly pushed back by hit-and-run Ukrainian units armed with Western-supplied weapons, Russian troops are shelling targets from afar, falling back on tactics they used in reducing cities to ruins in Syria and Chechnya.
Major Russian objectives remain unfulfilled. The capital, Kyiv, has been shelled repeatedly hit but is not even encircled.
More shelling and gunfire shook the city on Wednesday, with plumes of black smoke rising from the western outskirts, where the two sides battled for control of multiple suburbs.
Mayor Vitali Klitschko said at least 264 civilians have been killed in the capital since war broke out.
Mr Zelenskyy said 100,000 civilians remain in a city that had 430,000 people. It has been shattered by strikes from air, land and sea, and repeated efforts to get desperately needed food and other supplies to those trapped have often failed.
“They bombed us for the past 20 days,” said 39-year-old Viktoria Totsen, who fled from Mariupol to Poland.
“During the last five days, the planes were flying over us every five seconds and dropped bombs everywhere — on residential buildings, kindergartens, art schools, everywhere.”
Mr Zelenskyy, speaking on Tuesday in his nightly video address to his nation, said efforts to establish humanitarian corridors for Mariupol residents are almost all being “foiled by the Russian occupiers, by shelling or deliberate terror”.
He accused Russian forces of seizing one humanitarian convoy. Deputy Prime Minister Iryna Vereshchuk said the Russians were holding captive 11 bus drivers and four rescue workers along with their vehicles.
The head of the International Committee of the Red Cross travelled on Wednesday to Moscow for discussions with Russian officials on humanitarian aid, prisoners of war, and other matters.
“The devastation caused by the conflict in recent weeks, as well as eight years of conflict in Donbas, has been vast,” Red Cross president Peter Maurer said.
It is not clear how much of Mariupol is still under Ukrainian control. Fleeing residents say fighting continues street by street.
In their last update, over a week ago, Mariupol officials said at least 2,300 people had died, but the true toll is probably much higher.
Airstrikes in the past week destroyed a theatre and an art school where civilians were sheltering.
In the besieged northern city of Chernihiv, Russian forces bombed and destroyed a bridge that was used for aid deliveries and civilian evacuations, regional governor Viacheslav Chaus said.
Kateryna Mytkevich, who arrived in Poland after fleeing Chernihiv, wiped away tears as she spoke about what she had seen.
The city is without gas, electricity or running water, the 39-year-old said, and entire neighbourhoods have been destroyed.
“I don’t understand why we have such a curse,” she said.
Despite plenty of evidence to the contrary, the Kremlin spokesman insisted the military operation was going “strictly in accordance” to plans.
Russia wants to “get rid of the military potential of Ukraine” and “ensure that Ukraine changes from an anti-Russian centre to a neutral country,” Mr Peskov said.
Officially, Russia is calling the campaign a “special military operation”. It has effectively outlawed terms such as “invasion” and “war,” and police have arrested thousands of anti-war protesters.
But as casualties mount and quick victory is no longer in sight, Russia is having to work to shore up morale.
Under a law passed Wednesday, troops in Ukraine will get the same benefits as veterans of previous wars, including tax breaks, discounts on utilities and preferential access to medical treatment.
Western officials say Ukrainian resistance has halted much of Russia’s advance and Mr Putin’s forces are facing serious shortages of food, fuel and cold weather gear.
Russia’s military casualties are unclear but even conservative Western estimates are in the low thousands.
“We have seen indications that the Ukrainians are going a bit more on the offensive now,” Pentagon spokesman John Kirby said.
He said that was particularly true in southern Ukraine, including near Kherson.
But Russia’s far stronger, bigger military has many Western military experts warning against overconfidence in Ukraine’s long-term odds.
The Kremlin’s practice in past wars has been to grind down resistance with strikes that flattened cities, killing countless civilians and sending millions fleeing.
Talks to end the fighting have continued by video. Mr Zelenskyy said negotiations with Russia are going “step by step, but they are going forward.”
With no peace, those not yet fighting prepared to do so.
“Everything’s a best-seller these days,” said Zakhar Sluzhalyy, who owns a gun shop in the western city of Lviv.
“We’re defending our land,” he said.
“We’re fighting for our freedom and that of the rest of Europe.”