Russia’s strategy in its war on Ukraine is shifting toward a “slow annihilation” of the country’s military, US and other Western officials say.

CNN is reporting that the officials have warned Russia might instead focus on a bloody and deadly bombardment of cities and civilian targets, as its initial plans to quickly take Kyiv and other major Ukrainian cities have failed.

A week after Russian President Vladimir Putin launched an invasion of his neighbour, Ukrainian forces have so far been able to stave off the initial attacks.

They maintain control of the capital Kyiv and other major cities.

Eventually, however, officials say, Ukraine’s military will likely run short of supplies needed to keep up the fight.

The US has reportedly delivered hundreds of Stinger missiles to Ukraine in recent days. But the US and NATO have made it clear they will not commit troops to defending Ukraine.

Russia’s defence ministry claimed earlier on Thursday that its forces had taken control of the southern port city Kherson, home to about 250,000 people. Late on Wednesday, Kherson mayor Igor Kolykhayev said Russian troops were in the streets and had forced their way into the city council building.

“The [Russian] occupiers are in all parts of the city and are very dangerous,” Gennady Lakhuta, head of the regional administration, wrote on the messaging app, Telegram.

However, an adviser to Ukrainian President Volodymyr Zelensky said Ukrainian forces were still defending the Black Sea port city.

“We are a people who broke the enemy’s plans in a week,” Mr Zelensky said in a video address.

“These plans had taken years to write – they are mean, with hatred for our country, for our people.”

Despite the defiance, Ukraine forces remain massively outgunned and outmanned, as Russia moves to bring in heavier, more destructive weaponry. It is also increasingly targeting civilian infrastructure.

Bombing of Kharkiv, a city of 1.5 million people in eastern Ukraine, has left its centre a wasteland of ruined buildings and debris. Authorities said at least 25 people had been killed by shelling and air strikes in Kharkiv in the past 24 hours.

In Kyiv, thousands of women and children spent another night bunkered down in bomb shelters, basements and underground rail stations, as air raid sirens sounded repeatedly and the city braced for a major Russian attack.

Despite that, Western governments say Russia’s main advance on Kyiv has been largely stalled for days.

The United Nations Human Rights Office said it had confirmed the deaths of 227 civilians and 525 injuries during the conflict to midnight on March 1, mostly caused by “the use of explosive weapons with a wide impact area”.

It cautioned the real toll would be much higher due to reporting delays.

Russia’s defence ministry said 498 Russian soldiers had died and another 1597 had been wounded. It said more than 2870 Ukrainian soldiers and “nationalists” had been killed, Interfax news agency reported.

Ukraine said more than 7000 Russian soldiers had been killed so far and hundreds taken prisoner.

Neither side’s claims could be independently verified.

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Also on Thursday, Russia’s week-long invasion was denounced by the UN, and dozens of countries referred the Kremlin to be probed for potential war crimes.

The biggest attack on a European state since 1945 has already caused more than a million people to flee, launched a barrage of sanctions against Russia, and stoked fears of wider conflict in the West unthought-of for decades.

A UN resolution reprimanding Moscow was supported by 141 of the assembly’s 193 members, passed in a rare emergency session, a symbolic victory for Ukraine that increases Moscow’s international isolation.

“More is at stake even than the conflict in Ukraine itself,” US Secretary of State Antony Blinken said after the vote.

“This is a threat to the security of Europe and the entire rules-based order.”

A total of 38 countries have also referred Russia to the International Criminal Court, which probes alleged war crimes and crimes against humanity.

“We are crystal clear that Putin cannot commit these horrific acts with impunity,” British Prime Minister Boris Johnson said.

Russia has called its actions in Ukraine a “special operation” it said was not designed to occupy territory but to destroy its southern neighbour’s military capabilities and capture what it regarded as dangerous nationalists.

Diplomatic efforts to resolve the crisis have so far failed.

Russian Foreign Minister Sergei Lavrov said Moscow still sought Ukraine’s “demilitarisation” and that there should be a list of weapons that could never be deployed on Ukrainian territory. Moscow opposes Kyiv’s bid to join NATO.

A Ukrainian delegation had left for a second round of talks with Russian officials on a ceasefire, Ukrainian presidential adviser Mykhailo Podolyak told Reuters on Thursday.

President Volodymyr Zelenskiy has said Russia must stop bombing if it wanted to negotiate.

-with AAP