NATO Secretary General Jens Stoltenberg and British Prime Minister Boris Johnson have warned that the war in Ukraine could last years, as Russian forces expand their territory in the east of the former Soviet Union country.

“We must prepare for the fact that it could take years,” the NATO chief said in an interview with Germany’s Bild am Sonntag newspaper.

“We must not let up in supporting Ukraine. Even if the costs are high, not only for military support, but also because of rising energy and food prices.”

A NATO summit in Madrid later this month is expected to agree an assistance package for Ukraine that will help the country with the move from old Soviet-era weaponry to NATO standard gear, Stoltenberg said earlier this week.

UK Prime Minister Boris Johnson, who visited Kiev on Friday, also made similar comments and warned his country’s allies to prepare for a long war in Ukraine.

In an editorial for The Sunday Times newspaper, the premier stressed the need to avoid “Ukraine fatigue.”

“The worry that we have is that a bit of Ukraine fatigue is starting to set in around the world. It is important we show we are with them for the long haul and provide the strategic resilience that they need.”

“Time is the vital factor,” Johnson said. “Everything will depend on whether Ukraine can strengthen its ability to defend its soil faster than Russia can renew its capacity to attack. Our task is to enlist time on Ukraine’s side.”