They stressed that Russian forces were making “minimal progress” on land sea or air and were continuing to “suffer heavy losses”.
As the conflict entered its fourth week, they also stressed that Mr Putin’s troops had failed to seize any major cities despite the heavy bombardment of Mariupol, Kharkiv and other cities which has destroyed whole districts, according to footage and reports.
A theatre in the southern city of Mariupol, believed to have been housing more than 500 civilians including children, was hit by a powerful bomb on Wednesday, according to reports.
The number of casualties was not yet known.
In its latest intelligence briefing, the Ministry of Defence said: “The Russian invasion of Ukraine has largely stalled on all fronts.
“Russian forces have made minimal progress on land, sea or air in recent days and they continue to suffer heavy losses.
“Ukrainian resistance remains staunch and well-coordinated. The vast majority of Ukrainian territory, including all major cities, remains in Ukrainian hands.”
Some reports from America put the Russian military death toll at more than 7,000.
International outrage over Mr Putin’s invasion was spiralling as US and Ukrainian officials said civilians, including children, sheltering in the theatre, and people queuing for bread had been killed by Russian forces.
They are accused of dropping a powerful bomb on the theatre in Mariupol, which was reported to be housing at least 500 civilians.
Maxar Technologies, a private US company, distributed satellite imagery which it said was from March 14 and showed the word “children” in large Russian script painted on the ground outside the red-roofed building.
At least ten people were also killed in the northern city of Chernihiv as they queued for bread on Wednesday, according to US and local reports.
American President Joe Biden called the Russian leader a war criminal and there are fears that he could resort to even more barbaric tactics, including possibly the use of chemical weapons, as his ground forces fail to make progress.
The Kremlin, in words which flew in the face of the numerous reports of Russian setbacks, said the invasion was “going to plan”.
Moscow has yet to capture any of Ukraine’s biggest cities despite the largest assault on a European state since World War Two.
Russia-Ukraine Crisis: Ukrainian refugees on Polish border
More than three million Ukrainians have fled their homes.
Thousands of civilians are feared dead, as well as many Ukrainian soldiers.
In the capital Kyiv, at least one person died and three were wounded when a residential building was struck by a downed Russian missile, emergency services said on Thursday.
Moscow denies targeting civilians, despite the increasingly indiscriminate nature of missile attacks and shellings, and Russia’s defence ministry said its forces had not struck the building, RIA news agency said.
Thirteen buses carrying around 300 refugees from Mariupol have arrived in Russia’s Rostov region, Russia’s Interfax news agency cited the Russian defence ministry as saying on Thursday.
Ukraine’s armed forces are conducting small-scale counter attacks on several fronts and Russian troops have not been able to gain ground because of a lack of resources, Ukrainian presidential adviser Oleksiy Arestovych said on Wednesday.
There were signs of compromise and progress at ongoing talks between Russia and Ukraine.
Ukrainian President Volodymyr Zelensky said his country could accept international security guarantees that stopped short of its longstanding aim to join Nato. That prospect has been one of Russia’s primary concerns.
“My priorities during the negotiations are absolutely clear: the end of the war, guarantees of security, sovereignty, restoration of territorial integrity, real guarantees for our country, real protection for our country,” Mr Zelensky said in a video address released early on Thursday.
The Kremlin said negotiators were discussing a status for Ukraine similar to that of Austria or Sweden, both members of the European Union that are outside the Nato military alliance.
“Neutral status is now being seriously discussed along, of course, with security guarantees,” Russian Foreign Minister Sergei Lavrov said. “There are absolutely specific formulations which in my view are close to agreement.”
Vladimir Medinsky, Russia’s chief negotiator, told state TV: “Ukraine is offering an Austrian or Swedish version of a neutral demilitarised state, but at the same time a state with its own army and navy.”
Austria and Sweden, the biggest of six EU members outside Nato, both have small militaries that cooperate with the alliance.
The United States announced an additional $800 (£608) million in security assistance to Ukraine to fight Russia, with the new package including drones, anti-armour and anti-aircraft systems.
“More will be coming as we source additional stocks of equipment that ... we are ready to transfer,” Mr Biden said, later condemning Mr Putin as a “war criminal”.
Britain is supplying more weapons to Ukrainian forces including Starstreak misssiles and anti-tank weapons, with other Nato allies also providing more equipment.
The UN Security Council is due to vote on Friday on a Russian-drafted call for aid access and civilian protection, but diplomats say the measure is set to fail because it does not push for an end to the fighting or withdrawal of troops.