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US Secretary of State Antony Blinken on Sunday described “credible reports” of Russian war crimes in Ukraine and told of plans to tighten sanctions on Moscow and possibly help to arm Kyiv with fighter jets.
He praised Ukraine’s resistance and said Russian President Vladimir Putin was “destined to lose” the conflict.
"We've seen very credible reports of deliberate attacks on civilians, which would constitute a war crime," Mr Blinken told CNN as he visited Moldova, a small Eastern European nation rattled by Russian aggression.
"We are now in very active discussions with our European partners about banning the import of Russian oil to our countries, while of course at the same time maintaining a steady global supply of oil," Mr Blinken told NBC.
He said the US was working with Poland on plans to supply Ukraine with Russian-made MiG-29 fighters, which its pilots are trained to fly. In return, Warsaw would receive US-made F-16s.
“We are working with Poland as we speak to see if we can backfill anything that they provide to the Ukrainians,” Mr Blinken said.
Ukrainian police on Sunday described relentless Russian shelling and air raids in the north-east Kharkiv region, reporting many casualties, while the UN World Health Organisation said several Ukrainian clinics had been hit.
The number of refugees fleeing Ukraine was expected to reach 1.5 million on Sunday, as Kyiv pressed the West to tighten sanctions and provide more weapons to halt Russia's military advance.
Russia faces growing international condemnation for launching its invasion of Ukraine on February 24.
Attacks on schools, hospitals and residential blocks are set to be scrutinised by the International Criminal Court and a UN commission.
Mr Blinken spelt out the prospect of a lengthy war and widespread suffering as Russia laid siege to major Ukrainian cities.
“I think we have to be prepared for this to last for some time,” he said.
“But just winning a battle is not winning the war. Taking a city does not mean he's taking the hearts and minds of the Ukrainian people.”