Ukraine warned on Monday the humanitarian crisis in the pulverized city of Mariupol was now "catastrophic", while signaling grounds for compromise ahead of new face-to-face peace talks with Russia in Turkey.

About 20,000 Ukrainians have been killed in Russia's month-old invasion and 10 million have fled their homes, according to Kyiv, and several cities are still coming under withering bombardment.

After an apparent retreat in Moscow's war aims to focus on eastern Ukraine, Ukrainian intelligence said Russia could be bent on carving up the country into two entities like Korea.

Russia has de-facto control over the southern peninsula of Crimea that it annexed in 2014, and the self-proclaimed republics of Donetsk and Lugansk in the eastern Donbas region.

In the Lugansk city of Rubizhne, one person was killed and another wounded by overnight Russian bombardment, according to regional Ukrainian officials.

Further west near the capital Kyiv, Andrii Ostapets was determined to get back into his village of Stoyanka to bring food to his neighbors -- and to his cats, if still alive -- despite the threat of Russian snipers.

"We saw people killed, we saw burnt down houses, we lived through hell" when Russia occupied Stoyanka, the 69-year-old private museum owner told AFP, a week after fleeing the village.

But Ukrainian soldiers were pushing back the invaders at the village, Ostapets said. "The Russians have no chance to stay alive -- they can either surrender or die."

President Volodymyr Zelensky said the first round of in-person talks since March 10 -- due to open in Istanbul on Tuesday after near-daily video contacts -- must bring peace "without delay".

Red lines

Ukrainian "neutrality", and the future status of Donbas in line with Russian demands, could be in the mix for the Istanbul meeting as negotiators from both sides headed to Turkey on Monday.

"We understand that it is impossible to liberate all territory by force, that would mean World War III, I fully understand and realize that," Zelensky said.

But stressing his negotiating red lines, he added: "Ukraine's sovereignty and territorial integrity are beyond doubt. Effective security guarantees for our state are mandatory."

Humanitarian needs are direst in the southern port city of Mariupol, where Ukraine says about 170,000 civilians are encircled by Russian forces, with ever-dwindling supplies of food, water and medicine.

Ukraine's foreign ministry said the situation there was "catastrophic" and Russia's assault from land, sea and air had turned the city of 450,000 people "into dust".

France, Greece and Turkey are hoping to launch a mass evacuation of civilians out of Mariupol within days, according to French President Emmanuel Macron, who is seeking agreement from Russia's Vladimir Putin.

Ukraine decided against any humanitarian corridors on Monday because of potential "provocations" by the Russians along designated routes, Deputy Prime Minister Iryna Vereshchuk said.

Macron warned that any escalation "in words or action" could harm his evacuation efforts, after US President Joe Biden's shock declaration that Putin "cannot remain in power".

The ad-libbed remark on a visit to Poland sparked outrage in Moscow, and seemed to undercut Biden's own efforts for the West to present a united front.

Biden himself rowed back on Sunday, denying to reporters that he had been calling for regime change, while Britain and Germany have joined France in distancing themselves from the remark.

What does Putin want?

With Russia's much-larger military hampered by fierce Ukrainian resistance, the three days of Istanbul talks will test whether battlefield setbacks have tempered Moscow's demands.

Russia last week appeared to scale back its campaign when senior general Sergei Rudskoi said the "main goal" was now on controlling Donbas in the east.

For his part, Putin has avoided clearly defining the goals of his invasion, stating only that he wants to "demilitarize and denazify" Ukraine.

On March 10, the Turkish resort city of Antalya hosted the first talks between Russian Foreign Minister Sergei Lavrov and Ukrainian Foreign Minister Dmytro Kuleba since the start of the invasion.

There was no agreement then for a hoped-for ceasefire. Kremlin spokesman Dmitry Peskov said it was "important" that talks in person were resuming, after no "significant achievements or breakthroughs" in the previous virtual rounds.

Divided nation

Many in Ukraine remain suspicious, however, that Russia could use the talks as an opportunity to regroup and fix serious tactical and logistical problems bedeviling its military.

"After a failure to capture Kyiv and remove Ukraine's government, Putin is changing his main operational directions," intelligence chief Kyrylo Budanov said.

He was aiming now "impose a separation line between the occupied and unoccupied regions", the Ukrainian official said. "It will be an attempt to set up South and North Koreas in Ukraine."

The head of Ukraine's Lugansk separatist region says it may hold a referendum on becoming part of Russia.

But resistance in besieged Mariupol is the main obstacle preventing Moscow from gaining unbroken control of land from the Donbas to the Crimea.

Counterattacks

In the southern town of Mykolaiv, under heavy assault for weeks, the bombardments appeared to be easing.

That was a welcome respite for locals like 13-year-old Sofia, who suffered shrapnel injuries to her head during shelling in early March near Mykolaiv.

"Now I can move my arms and legs a little," she said, after undergoing three operations. "I still can't get up without my mother's help, but hopefully I can leave soon."

The frontlines appeared to have receded from Mykolaiv, with a counteroffensive being mounted in Kherson, about 80 kilometers (50 miles) to the southeast.

In Kherson, the only significant city the Russian army claims to have seized, about 500 people took part in anti-Russian demonstrations on Sunday.

Kyrylo, a paramedic, told AFP by phone that the Russians dispersed the peaceful rally with tear gas and stun grenades.