Russian strikes hit civilian targets in central Ukraine's Dnipro city on Friday, as Moscow's troops edged closer to Kyiv, where officials said the capital was being transformed into a "fortress".

Hundreds of thousands of civilians remained trapped and under fire in Ukrainian cities, including besieged Mariupol, after the first talks between Moscow and Kyiv's top diplomats ended Thursday without any progress.

In the early hours of Friday, Russian war planes carried out what appeared to be the first direct attack on Dnipro, killing one person, emergency services said in a statement.

Three air strikes hit a kindergarten, apartment building and a shoe factory, it said.

Meanwhile, two Ukrainian soldiers were killed and six wounded in Russian strikes on the Lutsk military airport in the northwest, local authorities said.

Russian forces are currently encircling at least four major Ukrainian cities, while the capital Kyiv is increasingly at risk of being surrounded.

The UN said some 2.5 million refugees have left Ukraine since Russia shocked the world by invading its neighbour on February 24.

'Nowhere to run'

The Ukrainian military warned Russia was trying to "block" Kyiv by taking out defences to the west and northwest of the capital, adding that there was also a risk to Brovary on the east.

Kyiv's mayor Vitali Klitschko said Thursday that half the city's population had fled and the capital "has been transformed into a fortress". 

"Every street, every building, every checkpoint has been fortified."

The northwest suburbs, including Irpin and Bucha, have endured days of heavy bombardment but Russian armoured vehicles are also advancing on the northeastern edge of Kyiv.

Ukrainian soldiers described fierce fighting for control of the main highway leading into the capital, and AFP reporters saw missile strikes in Velyka Dymerka just outside Kyiv's city limits.

"It's frightening, but what can you do?" said Vasyl Popov, a 38-year-old advertising salesman. 

"There is nowhere to really run or hide. We live here." 

Britain's defence ministry said in an intelligence update that "Russian forces are committing an increased number of their deployed forces to encircle key cities".

"This will reduce the number of forces available to continue their advance and will further slow Russian progress," it said.

Russian President Vladimir Putin on Friday backed plans to allow volunteers, including from abroad, to fight in what Moscow calls a "special military operation" in Ukraine. 

The Russian army this week admitted conscripts were taking part, after Putin previously said only "professional" soldiers were involved.

Desperation in Mariupol

The southern port city of Mariupol has suffered relentless bombardment, including on attempted aid deliveries, according to Ukraine's President Volodymyr Zelensky.

He said Moscow had launched a "tank attack" targeting a humanitarian corridor where he had dispatched a convoy to try to get food, water and medicine into the city.

The attack, which Zelensky described in a video statement as "outright terror", came a day after the bombing of a children's hospital there that local officials said killed three people, including a young girl.

Zelensky branded that attack a "war crime", a position backed by top Western officials, while Russia's army claimed the bombing was a "staged provocation" by Ukraine.

In a video, Mariupol mayor Vadym Boichenko said Russian warplanes had targeted residential areas in the city "every 30 minutes" on Thursday, "killing civilians, the elderly, women and children".

The situation in the city has been described as "apocalyptic", with more than 1,200 civilians killed in days of constant attacks, according to the mayor.

The International Committee of the Red Cross said some residents had started fighting for food, and many had run out of drinking water.

Yulia, a 29-year-old teacher who fled Mariupol, said her mother-in-law was still there, and told them "the attacks don't stop".

"There are many corpses on the street and nobody buries them," she told AFP.

Some humanitarian corridors out of cities under attack have held. 

Around 100,000 people have been able to leave the northeastern city of Sumy, the eastern city of Izyum, and areas northwest of Kyiv in the last two days, Ukrainian officials said.

Moscow said it would also open daily humanitarian corridors to evacuate civilians to Russian territory, but Kyiv has rejected routes leading to Russia.

'No progress' in Turkey talks

In Turkey, Ukraine's Foreign Minister Dmytro Kuleba said his talks on Thursday with his Russian counterpart Sergei Lavrov ended with "no progress", even on a 24-hour ceasefire.

Lavrov said the two sides would keep talking, but also insisted Russia's invasion was purely defensive.

Asked by a reporter if Moscow was planning to attack other nations, he insisted "we don't plan to attack other countries" and Russia "did not attack Ukraine".

He said Putin launched the operation as the situation in Ukraine "posed a direct threat to the Russian Federation".

Russia has also ramped up its claims about alleged biological weapons development in Ukraine, which Western officials have said could be an attempt to lay the ground for their possible use by Moscow's forces in the country.

On Friday, the UN Security Council will hold an urgent meeting on the subject at Moscow's request.

Western nations and allies have offered military and humanitarian support, and on Friday the US congress passed a budget, including $14 billion in humanitarian and military aid for Ukraine. 

But the US has ruled out enforcing a no-fly zone, and rejected a Polish plan to transfer fighter jets to Ukraine via a US air base for fear of being drawn directly into the conflict.

The State Department said Thursday that Washington would "continue to provide our Ukrainian partners with the surface-to-air systems that they need".

Moscow has warned supplying deadly weapons is "creating a colossal danger."

With global anger surging online as the war rages, Facebook said late Thursday it had temporarily eased its rules on violent speech to allow statements like "death to Russian invaders", while still barring threats against Russian civilians.