As talks ground on between Moscow and Kyiv in a bid to halt the devastation, the White House upped the ante by announcing President Joe Biden will visit Europe next week to shore up NATO's unity as war rages on its eastern flank.

The nearly three-week-old conflict has revived Cold War-level tensions between Moscow and the West and sent more than three million Ukrainians fleeing across the border to seek refuge in neighboring states.

On the ground in southern Ukraine the presidency reported a humanitarian breakthrough of sorts, with some 20,000 residents evacuating from the besieged port city of Mariupol where there is a critical lack of food, water and medicine.

Exhausted, shivering evacuees spoke of harrowing journeys out of the city of 400,000, and a stench of death on the streets.

"Sometimes bodies are in the street for three days. The smell is in the air and you don't want your children to smell it," said Dmytro, a refugee who arrived with his wife and two young children in Zaporizhzhia on Tuesday.

Despite the developments on evacuations, mixed messages emerged from the latest negotiations.

Both Moscow and Kyiv signalled progress, but a Ukrainian presidential aide, Mykhailo Podolyiak, cautioned that while "compromise" was possible, "fundamental contradictions" remained.

Kyiv was "not showing a serious commitment to finding mutually acceptable solutions," Russian President Vladimir Putin told European Council leader Charles Michel in a call Tuesday.

Russia extended its military onslaught elsewhere, including a huge strike on an eastern airport, while four people were killed in strikes on homes in the capital.

As a 35-hour curfew came into force in Kyiv, President Volodymyr Zelensky hosted the Polish, Czech and Slovenian prime ministers in the first visit to the city by foreign leaders since Russia's invasion.

"We have to halt this tragedy ... as quickly as possible," Polish Prime Minister Mateusz Morawiecki said on Facebook announcing his arrival, along with Czech premier Petr Fiala and Slovenia's Janez Jansa.

"They are shelling everywhere. Not only Kyiv but also the western areas," Zelensky told them, a video posted on his Telegram account showed.

- 'How many more' missiles? -

Zelensky earlier addressed a key concern used by Putin to justify the invasion, by saying Ukraine should accept it would not become a member of NATO.

"It's a truth and it must be recognised," Zelensky told military officials.

But in a virtual address to Canadian lawmakers, Zelensky pleaded again for NATO to establish a no-fly zone.

The appeal has so far not swayed Western nations, who fear the move could trigger a catastrophic war with nuclear-armed Russia. Zelensky made his disappointment clear.

"How many more of those missiles have to fall on our cities until you make this happen?" he asked.

Zelensky has another opportunity to press his case Wednesday, when he addresses the US Congress by videolink.

In Warsaw, Poland announced support for a different intervention: a NATO peace mission "protected by armed forces" to help Ukraine.

"This cannot be an unarmed mission," Vice Premier Jaroslaw Kaczynski said. "It must seek to provide humanitarian and peaceful aid."

According to the United Nations, nearly 1.4 million children have fled Ukraine since the conflict began on February 24 -- almost one child per second. The UN has reported 1,834 civilian casualties.

It has also warned of a mounting disaster in Mariupol, but the evacuation of thousands Tuesday raised hopes more residents can escape.

"This is the first time I have been able to breathe in weeks," said Mykola, whose family was among some 570 carloads that arrived in Zaporizhzhia, around 250 kilometres (155 miles) to the northwest.

Mykola, who asked not to give his full name, described heading off-road to avoid Russian checkpoints, and how he drove his wife and two young children through a minefield with help from Ukrainian troops.

"As we passed through, there was a burnt-out car," he said. "Soldiers said a woman had been blown up after she hit a mine just one hour before we got there."

- 'Are you alive?' -

Surrounded by Russian troops to the north and east, Ukraine's capital has been transformed into a war zone, with apartment blocks badly damaged by bombardment and half of its 3.5 million people now fled.

But Mykola Vasylinko, 62, said it was better than where he had come from -- the northern city of Chernigiv, which he said Russian bombardments are trying "to erase from the Earth's surface."

Four large blasts were heard from central Kyiv early Tuesday, sending columns of smoke skyward.

Fire swept through a 16-storey housing block, smoke billowing from the charred husk.

"At 4:20 am everything was very thunderous, crackling. I got up, my daughter ran to me with a question: 'Are you alive?'," Lyubov Gura, 73, told AFP.

- 'Massive destruction' -

Fox News announced Tuesday that one of its cameramen, French-Irish citizen Pierre Zakrzewski, and a Ukrainian producer, Oleksandra Kuvshynova, were killed when their vehicle was struck by incoming fire outside Kyiv a day earlier.

The news came after the Ukrainian parliament's human rights chief said three other journalists had been killed since the invasion began, including a US reporter shot Sunday in Irpin.

Overnight, Russian shelling also caused massive damage at the airport in the eastern city of Dnipro, authorities said.

"The runway was destroyed. The terminal is damaged. Massive destruction," said regional governor Valentin Reznichenko.

An AFP team saw plumes of black smoke spewing out of the airport but could not get closer as it was cordoned off by soldiers.

- Biden to Europe -

Outwardly the two sides are still far apart in negotiations, with Moscow demanding Ukraine turn away from the West and recognise Russia-backed breakaway regions.

Ukraine is pushing for a ceasefire and Russian troop withdrawal. On Tuesday, Zelensky sounded a note of cautious optimism.

"They have already begun to understand that they will not achieve anything by war," he said.

Western defence experts believe Russia's military needs time to regroup and resupply its troops, suggesting a possible slowdown in fighting.

In that context, Biden will use next week's trip to an emergency NATO summit to demonstrate Washington's "iron-clad" backing for its allies, the White House said, also attending an EU summit in Brussels to discuss the invasion.

The US had a clear message to Putin: "End this war," Secretary of State Antony Blinken said.

"It's going to end with an independent Ukraine," he told CNN, "and at some point it's going to end without Vladimir Putin."

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