Biden makes unannounced trip to Kyiv ahead of war's anniversary

US President Joe Biden made an unannounced visit to Kyiv on Monday in a show of support ahead of the one-year anniversary of Russia's invasion, promising President Volodymyr Zelenskiy that Washington would stand with Ukraine as long as it takes.

Air raid sirens blared across the Ukrainian capital as Biden visited Kyiv, although there were no reports of Russian missiles or air strikes.

"Your visit is an extremely important sign of support for all Ukrainians," Zelenskiy said, adding that he and Biden discussed long-range weapons during the visit.

The White House said Biden would announce more sanctions on Russia and military support for Ukraine including artillery ammunition, anti-armour systems and air surveillance radars.

Biden's visit came a day before President Vladimir Putin was due to make a major address, expected to set out Russia's aims for the second year of the invasion he launched on Feb. 24 last year.

The anniversary has taken on more than symbolic significance, becoming what the West views as motivation for the war's deadliest phase as Moscow hurls thousands of conscripts and mercenaries into a winter offensive.

Moscow received its own apparent signal of diplomatic support, with Chinese foreign minister Wang Yi expected in the Russian capital for talks. In public, China has so far remained neutral over the conflict despite signing a "no limits" friendship pact with Russia weeks before the invasion.

Washington has said in recent days it is concerned Beijing could begin supplying Moscow with arms. Chinese foreign ministry spokesperson Wang Wenbin said the United States was "in no position to make demands of China", and China's "comprehensive collaborative partnership with Russia" was a matter for two independent states.

Russia is trying to secure full control of two eastern provinces that form Ukraine's Donbas mining and industrial region. It has launched assaults at locations running from Kreminna in the north down to Vuhledar in the south, securing most of its recent gains around the mining city of Bakhmut.

Kyiv, which is absorbing a major influx of Western weaponry in coming months for a planned counter-offensive, has lately stuck mainly to defence on the battlefield, claiming to be inflicting huge casualties on the assaulting Russian forces.

"The situation is very complicated. And we are fighting. We are breaking down the invaders and inflicting extraordinarily significant losses on Russia," Zelenskiy said in his nightly video address.

"The more losses Russia suffers there, in Donbas - in Bakhmut, Vuhledar, Marinka, Kreminna - the faster we will be able to end this war with Ukraine's victory."

Britain's Ministry of Defence said Russia was taking huge casualties, including two elite brigades of thousands of marines probably rendered "combat ineffective" by high losses in failed attempts to storm Vuhledar, a heavily fortified Ukrainian bastion.

"The Russian forces are likely under increasing political pressure as the anniversary of the invasion draws near," it said, predicting Moscow would claim to have captured Bakhmut regardless of the situation on the ground. "If Russia's spring offensive fails to achieve anything, then tensions within the Russian leadership will likely increase."