That prospect is why Ukraine's NATO allies must continue supporting Kyiv militarily, Stoltenberg stressed.

"If Putin wins in Ukraine, there is real risk that his aggression will not end there. Our support is not charity. It is an investment in our security," he said.

"The only way to reach a just and lasting solution is to convince President Putin that they will not win on the battlefield," Stoltenberg continued.

"And the only way to ensure that President Putin realises that he is not winning on the battlefield is to continue to support Ukraine."

The NATO secretary general's comments came as support for further aid to Ukraine faltered in the United States and the European Union as a result of internal political manoeuvring.

Opposition Republicans in the United States have opposed to providing more US military aid, while in the EU, Hungary has held up approval of a 50-billion-euro ($55-billion) package to stabilise Ukraine's war-hit finances over the next three years.

EU leaders were gathered in Brussels on Thursday for a summit at which aid to Kyiv and Ukraine's bid to one day join the bloc were the headline issues.

Stoltenberg's warning also came at the same moment Putin said his troops were progressing along most of the front line in Ukraine.

"The situation of our troops is improving throughout," Putin asserted during a Moscow press conference.

Despite receiving weapons and ammunition from its allies, Ukraine has been unable over the past few months to make significant breakthroughs against entrenched Russian positions.