NATO Secretary-General Jens Stoltenberg urged Western countries to keep supporting Ukraine "even if the costs are high".
NATO's chief has warned that the war in Ukraine could last "for years" as President Volodymyr Zelenskyy vowed his forces would not give up the south of the country to Russia after he visited the frontline there.
NATO Secretary-General Jens Stoltenberg on Sunday urged Western countries must be ready to offer long-term military, political and economic support to Kiev during a grinding war.
"We must be prepared for this to last for years," Stoltenberg told German daily newspaper Bild.
"We must not weaken in our support of Ukraine, even if the costs are high – not only in terms of military support but also because of rising energy and food prices."
British Prime Minister Boris Johnson issued a similar warning, urging sustained support for Kiev or risk "the greatest victory for aggression" since World War II.
"Time is now the vital factor," Johnson wrote in an article for the Sunday Times after making his second visit to Kiev, calling for the West to ensure Ukraine has the "strategic endurance to survive and eventually prevail".
Ukraine has repeatedly urged Western countries to step up their deliveries of arms since the February 24 offensive, despite Russian warnings that it could trigger wider conflict.
The Ukraine war is fuelling not only a global food crisis but an energy crisis too.
Hit by punishing sanctions, Moscow has turned up the pressure on European economies by sharply reducing gas supplies, which has driven up energy prices.
Italian company Eni meanwhile joined a huge Qatari project to expand production from the world's biggest natural gas field, days after Russia slashed supplies to Italy.
Back in Kiev, thousands gathered to pay tribute to one young man – Roman Ratushny, a leading figure in Ukraine's pro-European Maidan movement, who was killed fighting Russians in the country's east earlier this month aged just 24.
In front of the coffin draped in a yellow and blue Ukrainian flag at the foot of a monument that overlooks the sprawling Independence Square in the capital, people of all ages saluted his memory.
"I think it is important to be here because he is a hero of Ukraine and we must remember him," Dmytro Ostrovsky, a 17-year-old high school student, told AFP news agency.
The loss put a human face on the shared grief of Ukrainians, as the bloodshed continues.
The worst of the fighting continues to be in the eastern industrial Donbass region, with battles raging in villages outside the city of Sievierodonetsk, which Russia has been trying to seize for weeks.