"The situation on the battlefield remains extremely serious. President Putin's aim to dominate Ukraine has not changed, and there are no indications that he is preparing for peace," Stoltenberg said in a recorded message to mark the second anniversary of the invasion.

"But we must not lose heart. Ukraine has demonstrated remarkable skill and fierce determination again and again."

The second anniversary of the invasion comes as Ukraine's outgunned troops are struggling to hold the front line and doubts swirl over future support from the United States.

Stoltenberg pointed to recent promises of military aid from NATO states "worth billions of dollars".

"These cover key capabilities like artillery ammunition, air defence, and combat boats. As well as F-16 equipment and spare parts, drones, and demining equipment.

"More support is on the way."

Stoltenberg said Putin had started his war to close the door to eventual NATO membership for Kyiv.

"But he has achieved the exact opposite: Ukraine is now closer to NATO than ever before," he said.

"Ukraine will join NATO. It is not a question of if, but of when. As we prepare you for that day, NATO will continue to stand with Ukraine. For your security, and for ours."