Slovakia on Friday became the second Nato country to say it would donate fighter jets to Ukraine, following an offer by Poland a day earlier.
Ukraine could receive as many as 13 Soviet-era MiG-29 jets in various states of readiness from Slovakia, Prime Minister Eduard Heger said.
Poland will provide another four MiG-29s and more will follow once they finish maintenance, President Andrzej Duda said.
Further allies have indicated they are ready to send such aircraft, the Polish government said this week, as Kyiv seeks air power to gain the upper hand over Russia.
Ukraine's Foreign Minister Dmytro Kuleba said Kyiv would remember who backed it "at the darkest moment of its history" when it comes to future diplomatic relations.
"War is a time when you have to make a choice. And every choice has been recorded," he told the BBC.
The fighter jet deliveries will cross a threshold in sending firepower to Ukraine, as many western allies have drawn the line at warplanes due to the risk of being drawn into a direct confrontation with Moscow.
Moscow said in response on Friday that the fighter jets given to Ukraine would be destroyed.
“The supply of this military equipment, as we have repeatedly said, will not change the outcome of the special military operation,” Kremlin spokesman Dmitry Peskov said.
Ukraine has its own stock of MiG-29s and the ageing aircraft do not meet the standard of more modern F-16s or similar models that Kyiv has craved most.
However, the new shipments could add to Ukraine’s fleet with operational jets or spare parts for its own damaged stock.
“Promises must be kept,” Slovakia's Mr Heger said on Friday, and when Ukraine's President Volodymyr Zelenskyy “asked for more weapons including fighter jets, I said we'll do our best.”
The US expressed support for the deliveries. National Security Council spokesman John Kirby called them “sovereign decisions,” although he said it did not “change our calculus” on President Joe Biden’s decision not to send F-16 fighters, for now.
“I’m not going to miss an opportunity to plug Poland, who is really punching above its weight when it comes to supporting Ukraine,” Mr Kirby said in Washington on Thursday.
The government in neighbouring Nato member Germany appeared caught off guard by Poland's announcement.
“So far, everyone has agreed that it’s not the time to send fighter jets,” German Defence Minister Boris Pistorius told reporters. “I don’t have any confirmation from Poland yet that this has happened.”
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Mr Zelenskyy and his military leadership have persistently demanded war planes since the first days of the war, saying they are essential to driving back the Russian invasion.
Those calls were renewed this year when western members of the transatlantic alliance pledged battle tanks to Ukraine, raising speculation that fighter jets would be next.
Mr Duda said Poland received the aircraft from East Germany. The planes — a dozen or more at the end of their technical lifespan — will now strengthen Ukraine’s fleet of Soviet-era jets.
MiG deliveries will not jeopardise Poland’s security as the nation’s air force upgrades to Korean FA-50 and US-made F-35 combat jets, he said.
Slovakia's 13 MiGs are in various states of readiness and were grounded last August after the termination of a maintenance agreement with Russia. They will be offered to Ukraine for servicing in its own factories.
No other allies have yet come forward with a contribution pledge.
Other Nato members with MiG-29 jets in their inventories include Bulgaria, Croatia and Romania.
