BRUSSELS, Dec 17 (AFP) - Russia is readying for a protracted war in Ukraine and Kyiv's NATO backers must keep sending weapons until President Vladimir Putin realises he "cannot win on the battlefield", alliance chief Jens Stoltenberg told AFP on Friday (Dec 16).

Almost 10 months into Moscow's invasion, Kyiv's forces have inflicted a succession of defeats on the Kremlin that have seen swathes of territory liberated.

But NATO Secretary General Stoltenberg warned there was no sign Putin had "given up his overall goal of controlling Ukraine".

"We should not underestimate Russia. Russia is planning for a long war," Stoltenberg said in an interview.

"We see that they are mobilising more forces, that they are willing to suffer also a lot of casualties, that they are trying to get access to more weapons and ammunition," he added.

"We have to understand that President Putin is ready to be in this war for a long time and to launch new offensives."

NATO allies, led by the United States, have sent billion of dollars of weaponry to Ukraine that have helped it turn the tables in the conflict and put Moscow on the back foot.

"Most likely this war will end at the negotiating table, as most wars do," Stoltenberg said, insisting any solution should ensure "Ukraine prevails as a sovereign, independent nation.

"The fastest way to achieve that is to support them militarily so President Putin understands that he cannot win on the battlefield but has to sit down and negotiate in good faith."