With the diplomatic and economic costs mounting for Russian President Vladimir Putin, Ukrainian leader Volodymyr Zelensky extolled his own people's "heroic" resistance.

He said that around 9,000 Russian soldiers had been killed since the invasion began eight days ago. Announcing its own toll for the first time, Moscow said it had lost 498 troops.

"We are a nation that broke the enemy's plans in a week," President Zelensky said in a video posted on the Telegram messaging service.

"Plans written for years: sneaky, full of hatred for our country, our people."

However, after a three-day siege that left Kherson short of food and medicine, Ukrainian officials conceded the loss of the Black Sea city of 290,000 people.

While a huge military column is stalled north of Ukraine's capital Kyiv, Russian troops have been advancing on the southern front, and are besieging the important port city of Mariupol east of Kherson. 

The Russians "just wanted to destroy us all", Mariupol's mayor Vadym Boychenko said, accusing their forces of shooting at residential buildings. 

Ukraine's military authorities said residential and other areas in the eastern city of Kharkiv had been "pounded all night" by indiscriminate shelling, which UN prosecutors are investigating as a possible war crime.

Oleg Rubak's wife Katia, 29, was crushed in the rubble of their family home in Zhytomyr, 93 miles west of Kyiv, by a Russian missile strike.

"One minute I saw her going into the bedroom, a minute later there was nothing," Rubak, 32, told AFP, standing by the ruins in jogging bottoms and a fleece.

"I hope she's in heaven and all is perfect for her."

He sobbed, apologized and continued: "I want the whole world to hear my story."

Junk status

The UN says the war has displaced more than 1 million people, after Putin launched his offensive in a bid to demilitarize Ukraine and depose Zelensky's Western-leaning government.

But the Russian president now finds himself an international pariah, his country the subject of swingeing sanctions that sent the ruble into further freefall on currency markets Thursday.

Russia's central bank -- whose foreign reserves have been frozen in the West -- imposed a 30-percent tax on all sales of hard currency, following a run on lenders by ordinary Russians.

The unfolding financial costs were underlined as ratings agencies Fitch and Moody's slashed Russia's sovereign debt to "junk" status. 

Its sporting isolation worsened as the International Paralympic Committee staged an abrupt U-turn and banned Russian and Belarusian athletes from competing in the Beijing Winter Games.

The UN General Assembly voted 141-5 to demand that Russia "immediately" withdraw from Ukraine. Only four countries supported Russia – Belarus, Eritrea, North Korea and Syria – while China abstained.

Russian Foreign Minister Sergei Lavrov accused Western politicians of considering nuclear war, saying it was in their heads "that the idea of a nuclear war is spinning constantly, and not in the heads of Russians".

The invasion has triggered a dramatic realignment of security policy in Europe, with NATO reinforcing its eastern flank and Germany planning a big increase in military spending. 

The German government is planning to deliver another 2,700 anti-air missiles to Ukraine, a source said.

US Secretary of State Antony Blinken warned the human costs were already "staggering", accusing Russia of attacking places that "aren't military targets".

"Hundreds if not thousands of civilians have been killed or wounded," said Blinken, who will travel to eastern Europe next week to shore up support for Ukraine – and for efforts to secure a ceasefire.

Kyiv is sending a delegation to Thursday's ceasefire talks, at an undisclosed location on the Belarus-Poland border but has warned it will not accept "ultimatums".

A first round of talks on Monday, also in Belarus, yielded no breakthrough.