Russia is preparing for an extended war so Nato must get ready "for the long haul" and support Ukraine for as long as it takes, top military chiefs from across Europe have been told.
Speaking at the opening of the military chiefs' meeting in Brussels on Wednesday, Nato's deputy secretary-general Mircea Geoana said alliance nations must invest more in defence, ramp up military industrial manufacturing, and harness new technologies to prepare for future wars.
As Russia's war on Ukraine nears the one-year mark, Nato chiefs are expected to discuss how allies can expand the delivery of weapons, training and support to Ukraine in the coming months, and how they can further shore up their own defences.
"We have no indication that Putin's goals have changed," said Mr Geoana, adding that Russia has mobilised more than 200,000 additional troops.
"So we must be prepared for the long haul. 2023 will be a difficult year and we need to support Ukraine for as long as it takes."
US Army General Mark Milley, chairman of the Joint Chiefs of Staff, had a meeting on Tuesday with Ukraine's chief military officer, General Valerii Zaluzhnyi, at an undisclosed location in south-eastern Poland, near the Ukrainian border.
He was expected to relay General Zaluzhnyi's concerns to the other military leaders at the meeting.
Gen Milley's meeting with Gen Zaluzhnyi was quickly arranged when it became clear on Monday that the Ukrainian chief would not be able to attend the Nato sessions in person.
He is expected to attend by video-link on Thursday.
General Milley's spokesman, Colonel Dave Butler, said the chairman planned to describe to the Nato chiefs "the tactical and operational conditions on the battlefield and what the military needs are for that".
Admiral Rob Bauer, chairman of the Nato Military Committee, told the gathering of leaders that the alliance must continue to transform to meet future fights.
"The war has also shown us that you have to be able to fight tomorrow's battles, as well as yesterday's battles, today," said Adm Bauer, from the Royal Netherlands Navy.
"Modern warfare is just as much about bits and bots as it is about mud and blood."
He noted that the Nato Russia Council held its last meeting about a year ago.
"Back then we were still able to sit around the same table," he said, adding that now, after Russia's brutal invasion and war in Ukraine, "the world is a different place".
He added that Nato has proved it can quickly increase and shift its military presence when and where needed. He repeated the alliance's commitment that members are prepared to support Ukraine for as long as needed.
Mr Putin, Adm Bauer said, "under-estimated the scale and bravery of the Ukrainian people, armed forces and leadership, and under-estimated our unity and our solidarity with Ukraine".
That alliance support, he said, has made a difference on the battlefield, and it will continue.