Russia signaled Friday it may dial back its war aims to focus on eastern Ukraine after failing to break the nation's resistance in a month of fighting and attacks on civilians, including up to 300 feared killed in the bombing of a theatre.

The possible shift came as President Joe Biden visited elite US troops serving with NATO just across the border in Poland and France's Emmanuel Macron proposed an internationally backed evacuation of civilians trapped in the bombarded city of Mariupol.

Russian President Vladimir Putin had ordered the February invasion to destroy Ukraine's military and topple pro-Western President Volodymyr Zelensky, bringing the country under Russia's sway.

However, Sergei Rudskoi, a senior general, suggested a considerably reduced "main goal" of controlling Donbass, an eastern region already partly held by Russian proxies.

His surprise statement came as a Western official reported that a seventh Russian general, Lieutenant General Yakov Rezanstev, had died in Ukraine and that a colonel had been "deliberately" killed by his own demoralized men.

Complicating Moscow's challenges, invasion troops were facing a counteroffensive in Kherson, the only major Ukrainian city under Russian control.

Visiting Rzeszow, about 80 kilometres (50 miles) from Ukraine, Biden praised Ukraine's "incredible" resistance, comparing the conflict to a bigger version of communist China's 1989 crushing of protests in Tiananmen Square.

Biden told soldiers from the 82nd Airborne Division that the struggle in eastern Europe represents a historic "inflection point".

"Are democracies going to prevail... or are autocracies going to prevail? And that's really what's at stake," Biden said.

The US leader was briefed on the humanitarian situation, with more than 3.7 million refugees fleeing Ukraine, most of them into Poland.

Earlier, he ended a trip to Brussels for meetings with Western allies by announcing new measures to help the European Union shed dependence on imported Russian energy.

The plan is part of a sea change in the West, which for years has shrunk from direct confrontation with the Kremlin, but now seeks to make Putin a pariah.

Russia's far bigger military continued to combat determined Ukrainian defenders who are using Western-supplied weapons, from near the capital Kyiv to Kharkiv, the Donbass region and the devastated southern port city of Mariupol.

Authorities said they fear some 300 civilians in Mariupol may have died in a Russian air strike on a theatre being used as a bomb shelter last week.

The theatre was targeted despite the word "children" being written large in Russian on the ground outside, so as to be visible to pilots.

Russian forces hammering Mariupol's out-gunned defenders consider the city a lynchpin in their attempt to create a land corridor between the Crimea region, which Moscow seized in 2014, and the Donbass.

France's Macron announced a bold plan with Turkey and Greece to evacuate "all those who wish to leave Mariupol", adding he would discuss it with Putin soon.

One Mariupol resident who already left the city, 33-year-old Oksana Vynokurova, described leaving behind a hellscape.

"I have escaped, but I have lost all my family. I have lost my house. I am desperate," she told AFP after reaching the western city of Lviv by train.

"My mum is dead. I left my mother in the yard like a dog, because everybody's shooting."

Zelensky said in a video statement Friday that despite thousands of evacuations from Mariupol, "the situation in the city remains tragic".

Russia's army was predicted by some to roll across Ukraine with little resistance.

However, Putin's military has exhibited poor discipline and morale, faulty equipment and tactics, as well as brutality toward civilians.

Amid heavy censorship, Russian authorities Friday gave only their second official military death toll since the start of the invasion, at 1,351.

This is far below Western estimates, with one senior NATO official saying between 7,000 and 15,000 Russian soldiers have died.

Rudskoi's announcement of a pivot to the battle for eastern Ukraine was accompanied by claims of success.

He said Ukraine's military has been severely degraded and that Russia hadn't seized cities in order to "prevent destruction and minimise losses among personnel and civilians".

But his reference to plans for a "liberation" of the Donbass region could lay the groundwork for the Kremlin to focus on an easier campaign that can be sold to Russians as a victory.

Meanwhile, Ukrainians are mounting an increasingly aggressive defence and in places even taking back ground.

Britain's defence ministry said Ukrainian counter-attacks are underway near Kyiv and a Pentagon official said Ukrainian forces were also attempting to recapture Kherson, the only major city held by Russian invasion troops.

Ambulances rushed more people out of the devastated commuter town of Irpin, northwest of Kyiv on Friday, AFP journalists said, as Ukrainian forces tried to push back Russian forces.

A giant pall of black smoke rose from the direction of Irpin, scene of some of the war's heaviest fighting, while shell explosions echoed off nearby apartment blocks.

Paramedics took one elderly woman with a waxen face out of an ambulance on a bloodstained stretcher, as the sound of blasts and air raid sirens could be heard late into the night across the capital.

As the Russian war machine stumbles, Western officials are warning Putin could resort to chemical weapons.

In Brussels on Thursday for NATO, EU and G7 summits, Biden said the transatlantic alliance would "respond" if Putin does use chemical warfare -- though a top advisor stressed the US itself "has no intention of using chemical weapons".

Kremlin spokesman Dmitry Peskov accused Biden of seeking to "divert attention".

And Putin, whom Biden again branded a "war criminal", gave a speech Friday saying Russia was the victim, comparing Western boycotts to "Nazis in Germany".

Earlier Friday, Biden and EU commission chief Ursula von der Leyen announced a joint energy task force seeking a way for Europe to break its energy dependence on Russia.

Germany, Moscow's biggest customer in Europe, said it would halve Russian oil imports by June and end all coal deliveries by autumn.

However, von der Leyen said the campaign is working, and "draining Putin's resources to finance this atrocious war".