The conflict between Russia and Ukraine could last for years, NATO Secretary-General Jens Stoltenberg told German newspaper Bild in an interview published on Saturday, while reaffirming the alliance's support for Kyiv.

"We must prepare for the fact that it could take years. We must not let up in supporting Ukraine," Stoltenberg said. "Even if the costs are high, not only for military support, also because of rising energy and food prices."

Stoltenberg also reiterated that although NATO will support Ukraine in its self-defense, it will not send its soldiers into direct conflict.

He told the newspaper that NATO members will announce that Russia is no longer a partner for the alliance but a threat to its security, as well as to "peace and stability", at the Madrid summit later this month.