"We must prepare for the fact that it could take years," Nato's secretary general, Jens Stoltenberg, said in an interview with the German newspaper Bild on Sunday. "We must not let up in supporting Ukraine."

 

The British Prime Minister, Boris Johnson, echoed Stoltenberg's comments. "I am afraid that we need to steel ourselves for a long war," he said, adding that it was necessary "to enlist time on Ukraine's side," The Guardian reported.

 

It came as the new head of the British army said British troops must prepare "to fight in Europe once again". "There is now a burning imperative to forge an army capable of fighting alongside our allies and defeating Russia in battle," Gen Sir Patrick Sanders said, writing to his charges about the challenges they face, The Guardian reported.

 

The statements suggest the west believes Ukraine cannot achieve a rapid military breakthrough despite the anticipated arrival of fresh Nato-standard arms, while officials in the country have continued to call for rapid help.

 

Ukraine's forces remain on the defensive in the eastern Donbas region, where fighting continues in Sievierodonestsk. Serhiy Haidai, the governor of the Luhansk region, said Russia was massing forces in an attempt to take full control of the city after weeks of fighting.

 

"Today, tomorrow, or the day after tomorrow, they will throw in all the reserves they have... Because there are so many of them there already, they're at critical mass," Haidai told Ukrainian television, The Guardian reported.

 

Russia already controls most of Sievierodonetsk, Haidai said on Sunday morning, and if Ukrainian forces lose the city, fighting is expected to focus on neighbouring Lysychansk.