Russia wants to split Ukraine into two, as happened with North and South Korea, Ukraine's military intelligence chief said, vowing "total" guerrilla warfare to prevent a carve-up of the country.

President Volodymyr Zelensky urged the West to give Ukraine tanks, planes and missiles to help fend off Russian forces.

Top American officials sought on Sunday to clarify that the United States does not have a policy of regime change in Russia, after President Joe Biden said at the end of a speech in Poland on Saturday that Russian leader Vladimir Putin "cannot remain in power".

US Secretary of State Antony Blinken said Biden had simply meant Putin could not be "empowered to wage war" against Ukraine or anywhere else.

After more than four weeks of conflict, Russia has failed to seize any major Ukrainian city and signalled on Friday it was scaling back its ambitions to focus on securing the Donbass region of eastern Ukraine, where Russian-backed separatists have been fighting the Ukrainian army for the past eight years.

A local leader in the self-proclaimed Luhansk People's Republic said on Sunday the region could soon hold a referendum on joining Russia, just as happened in Crimea after Russia seized the Ukrainian peninsula in 2014.

Crimeans voted overwhelmingly to break with Ukraine and join Russia - a vote that much of the world refused to recognise.

"In fact, it is an attempt to create North and South Korea in Ukraine," Kyrylo Budanov, the head of Ukrainian military intelligence, said on Sunday in a statement, referring to the division of Korea after WWII.

He predicted Ukraine's army would repel Russian forces.

"In addition, the season of a total Ukrainian guerrilla safari will soon begin. Then there will be one relevant scenario left for the Russians, how to survive," he said.

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Ukraine's foreign ministry spokesperson also dismissed talk of any referendum in eastern Ukraine.

"All fake referendums in the temporarily occupied territories are null and void and will have no legal validity," Oleg Nikolenko told Reuters.

'Cruel and senseless' conflict

Moscow says the goals for what Putin calls a "special military operation" include demilitarising and "denazifying" its neighbour. Ukraine and its Western allies call this a pretext for unprovoked invasion.

A Ukrainian negotiator said the two sides would hold talks this week in Turkey, a NATO member that has good relations with both Russia and Ukraine. A Russian negotiator confirmed in-person talks early this week, without giving further details.

Ukraine has described previous negotiations, some of which have taken place in Russian ally Belarus, as "very difficult".

The invasion has devastated several Ukrainian cities, caused a major humanitarian crisis and displaced an estimated 10 million people, nearly a quarter of Ukraine's population.

Tatyana Manyek, who crossed the Danube by ferry into Romania on Sunday with other refugees, said people in her home city of Odesa were "very afraid" but she would have stayed were it not for her daughter.

"It would be very difficult to provide the child with basic living conditions. That's why we decided to leave," she said, clutching a pet dog in her arms.

In his Sunday blessing, Pope Francis called for an end to the "cruel and senseless" conflict.

Call for weapons

Zelensky demanded in a late-night television address on Saturday that Western nations hand over military hardware that was "gathering dust" in stockpiles, saying his nation needed just 1 percent of NATO's aircraft and 1 percent of its tanks.

Western nations have given Ukraine anti-tank and anti-aircraft missiles as well as small arms and protective equipment, without offering heavy armour or planes.

Ukrainian Interior Ministry adviser Vadym Denysenko said Russia had started destroying Ukrainian fuel and food storage centres. Appearing to confirm that, Russia said its missiles had wrecked a fuel deposit on Saturday as well as a military repair plant near the western city of Lviv.

Ukraine was mounting small counter-offensive actions as Russian forces try to encircle its forces in eastern Ukraine, a Ukrainian presidential advisor said.

The United Nations has confirmed 1119 civilian deaths and 1790 injuries across Ukraine but says the real toll is likely to be higher. Ukraine said on Sunday 139 children had been killed and more than 205 wounded so far in the conflict.

Ukraine and Russia agreed two "humanitarian corridors" to evacuate civilians from frontline areas on Sunday, including allowing people to leave by car from the southern city of Mariupol, Ukrainian Deputy Prime Minister Iryna Vereshchuk said.

The encircled port, located between Crimea and eastern areas held by Russian-backed separatists, has been devastated by weeks of heavy bombardment. Thousands of residents are sheltering in basements with scarce water, food, medicine or power.