But in an interview with the BBC, Mr GeoanÄ predicted “the war would probably take longer”.
He added: “It could be weeks, could be months, could be even years — it depends on a lot of factors. But, in the end, probably this will be fought and won, hopefully, by Ukraine on the battlefield.”
The warning came as it emerged that two British aid volunteers had gone missing. Dominic Byrne, of the non-profit Presidium Network, said contact was lost on Monday.
Presidium said the men were working as part of a project to help provide food, medical supplies and evacuation support.
Mr Byrne told BBC Breakfast that it was 90 per cent likely the two Britons had been captured by Russian forces.
It is believed they were taken while trying to carry out an independent evacuation near the southern city of Zaporizhzhia.
International Trade Secretary Anne-Marie Trevelyan said the Foreign Office was “doing all it can to support and identify these two people”.
But, coming a day after the Government confirmed that British national Scott Sibley had been killed in Ukraine and another was missing, the minister added: “As we’ve set out right from the beginning, we don’t want British nationals to go and fight.”
In other developments in the conflict:
Ukraine president Volodymyr Zelensky accused Russia’s leadership of trying to “humiliate the UN and everything that the organisation represents” after Russian forces launched missile strikes on Kyiv while secretary-general Antonio Guterres was in Ukraine’s capital.
Mr Zelensky’s office said it was hopeful civilians would today be evacuated from the vast Azovstal steel plant, the last part of the besieged port city of Mariupol not to fall to the Russians. Hundreds of troops and civilians are believed to be trapped in the complex.
Foreign Secretary Liz Truss announced that the UK will deploy a team of war crimes experts to support Ukraine with investigations into Russian atrocities.
UK defence chiefs said Russian forces were paying a high price for “limited” territorial gains in the battle for the Donbas. In its latest intelligence update, the Ministry of Defence said there had been heavy fighting around the cities of Lysychansk and Severodonetsk in the Luhansk region.
Meanwhile, around 8,000 British troops will take part in exercises in eastern Europe in one of the largest deployments since the Cold War.
Dozens of tanks will be deployed to countries ranging from Finland to North Macedonia this summer under plans that have been enhanced since the war in Ukraine.
Defence Secretary Ben Wallace said it was “show of solidarity and strength”.