LVIV: Talks between Ukraine and Russia's foreign ministers on Thursday failed to bring any respite in the conflict as hundreds of thousands of civilians remained trapped in Ukrainian cities sheltering from Russian air raids and shelling.

With Russian President Vladimir Putin's war in Ukraine entering its third week, officials in Mariupol said Russian warplanes again bombed the southern port city where a maternity hospital was pulverised the day before.

Putin, facing global condemnation for the assault and increasingly isolated, said Russia would ultimately emerge stronger after overcoming the difficulties caused by international economic sanctions.

He told a government meeting there had been no alternative to what Russia calls its special military operation in Ukraine.

"There are some questions, problems and difficulties but in the past we have overcome them and we will overcome them," he said.

But the British government blacklisted more Russian oligarchs on Thursday, including Britain's best-known, Roman Abramovich, owner of Chelsea soccer team.

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Despite Putin's defiance, the invasion has so far failed to reach its stated objectives but it has caused thousands of deaths and more than two million people to flee the country, while several cities are under siege.

No breakthrough

Russian Foreign Minister Sergei Lavrov and Ukraine's Dmytro Kuleba met in Turkey on Thursday in the highest-level talks since the conflict began.

Kuleba said afterwards that Lavrov had refused to promise to hold fire so aid could reach civilians and hundreds of thousands of civilians trapped in Mariupol and elsewhere could be evacuated along humanitarian corridors.

At a separate news conference, Lavrov showed no sign of making any concessions, saying the operation was going to plan and repeating Moscow's accusations that Ukraine posed a threat to Russia.

A ceasefire was not meant to be on the agenda at Thursday's talks in Antalya, Lavrov said.

Aid agencies say humanitarian help is most urgently needed in Mariupol, where residents are running out of food, water and power. The capture of Mariupol would allow Russia to link up pro-Moscow enclaves in the east and the Russian-annexed Crimea to the southwest.

Attempts to send aid and evacuation convoys have failed for six days.

Russian warplanes were targeting convoy routes on Thursday, said Petro Andrushenko, an adviser to Mariupol's mayor.

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"Air strikes started from the early morning. Air strike after air strike. All the historic centre is under bombardment," he told Reuters by phone.

"They want to absolutely delete our city, delete our people. They want to stop any evacuation."

Lavrov said the hospital struck on Wednesday had stopped treating patients and had been occupied by Ukrainian "radicals". The Kremlin said the incident was being investigated.

Russia's Defence Ministry later denied having bombed the hospital, accusing Ukraine of a "staged provocation" there.

Ukraine's President Volodymyr Zelenskiy asked on Wednesday: "What kind of country is this, the Russian Federation, which is afraid of hospitals, is afraid of maternity hospitals, and destroys them?"

In Brussels, European Commission President Ursula von der Leyen said the hospital attack could constitute a war crime and should be investigated, calling it "inhumane, cruel and tragic".

Lavrov accused Western countries of inflaming the situation by arming Ukraine.

Asked if the conflict could lead to nuclear war, he said: "I don't want to believe, and I do not believe, that a nuclear war could start."

Russia says its offensive is aimed at disarming its neighbour and dislodging leaders it calls neo-Nazis. Kyiv and its Western allies say this is a baseless pretext to invade a democratic country of 44 million people.

But so far Russian forces have failed to crush Ukraine’s military and remove its leaders, while Zelenskiy has rallied his people and Western military aid has poured across the Polish and Romanian borders.

Russian troops have advanced in the south but have yet to capture a city in the north or east.

Western countries have said they believe that after a planned lightning strike on the capital Kyiv failed in the early days of the war Moscow has turned to tactics involving far more destructive assaults.

Britain said on Thursday a Russian column northwest of Kyiv had made little progress in over a week and was suffering losses.

The Western-led sanctions designed to cut off the Russian economy and government from international markets have bitten hard, with the rouble plunging.

'Barbaric'

The pounding of Mariupol underscored U.S. warnings that the biggest assault on a European state since 1945 could become increasingly attritional after Russia's early setbacks.

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Russia has repeatedly pledged since Saturday to halt firing so at least some trapped civilians could escape Mariupol. Both sides have blamed the other for the failure of the evacuations.

Half of the more than 2 million total refugees from Ukraine are children. The International Committee of the Red Cross said houses had been destroyed all across Ukraine.

Survivors of the worst-hit cities are among the refugees, many suffering physical injuries and psychological trauma.

At the Polish border, Valera, a carpenter in his 50s, looked on nervously as his daughter Anna, 24, was borne on a stretcher.

It was two days since they had left the eastern city of Kharkiv, where Anna, who has cerebral palsy, broke her leg as they ran to a bomb shelter.

"There is positional fighting during the day, air raids in the evenings, they are shelling from everything, fighter aircraft," he said. "The centre is ruined, the outskirts have already been bombed."

He was one of few men to cross from Ukraine as those of conscription age are generally obliged to stay.

Zelenskiy's chief economic adviser said Russian forces had so far destroyed at least $100 billion worth of Ukrainian infrastructure, buildings and other physical assets.

The war has caused 50% of Ukrainian businesses to shut down completely, while the other half are operating at well below capacity, Oleg Ustenko told an online event.