Kremlin spokesman Dmitry Peskov reiterated that Russian President Vladimir Putin remains open to talks but the demand from Ukraine and its allies that it first withdraws its troops from Ukraine is unacceptable.

Peskov's comments came as Putin spoke on the phone on Friday morning with German Chancellor Olaf Scholz.

Scholz's office said he made clear to Putin "that there must be a diplomatic solution as quickly as possible, which includes a withdrawal of Russian troops".

On Thursday, US President Joe Biden also indicated he would be willing to talk with Putin if he demonstrated that he seriously wanted to end the invasion and pull out of Ukraine.

A statement issued by the Kremlin after the phone call with Scholz said Putin again blamed the US and its allies for encouraging Ukraine to prolong the war by supplying it with weapons.

Putin also said recent crippling Russian strikes on Ukraine's infrastructure were "forced and inevitable" after Ukraine allegedly bombed a key bridge to the Crimean peninsula - which Russia seized from Ukraine in 2014 - and energy facilities.

Russian forces have been bombarding Ukraine's critical infrastructure since October, leaving millions without electricity amid cold winter weather.

Scholz's office said that in the phone conversation with Putin he "condemned in particular the Russian air attacks on civilian infrastructure" in Ukraine and said Germany was committed to continuing to help Ukraine defend itself.

Russian forces kept up rocket attacks on infrastructure and airstrikes against Ukrainian troop positions along the contact line, the Ukrainian general staff said on Friday, adding that the Russian military push has focused on a dozen towns including Bakhmut and Avdiivka.

A top adviser to Ukrainian President Volodymyr Zelenskiy, citing military chiefs, said that since Russia invaded in February, 10,000 to 13,000 Ukrainian soldiers have been killed in action.

It was a rare comment on Ukraine's military casualties and far below estimates from foreign leaders.

"We have official figures from the general staff, we have official figures from the top command and they amount to between 10,000 and 12,500-13,000 killed," the adviser, Mykhailo Podolyak, said late on Thursday on Channel 24 TV.

He also said civilian casualties were "significant".

The Ukrainian military has not confirmed such figures and it was a rare instance of a Ukrainian official providing such a count.

The last dates back to August when the head of the armed forces said nearly 9000 military personnel had been killed.

In June, Podolyak said up to 200 soldiers were dying each day in some of the most intense fighting and bloodshed so far in the war.

On Wednesday, Ursula von der Leyen, the president of the European Union's executive Commission, said 100,000 Ukrainian troops had been killed before her office corrected her comments - calling them inaccurate and saying that the figure referred to both dead and injured.

Australian Associated Press