"A weaker Putin is a greater danger. So we have to be very much aware of the consequences," Josep Borrell said ahead of a gathering of EU leaders in Brussels.

"Until now we were looking at Russia as a threat because it was a lot of force and force has been used in Ukraine. Now we have to look at Russia as a risk because of internal instability," Borrell told reporters.

Putin confronted the biggest challenge since he came to power in 1999 after Wagner chief Yevgeny Prigozhin spearheaded a mutiny at the weekend.

EU chief Ursula von der Leyen said the rebellion would lead to "aftershocks" and insisted it was "important that we double down on the support for Ukraine, be it military capability or financial support".

NATO chief Jens Stoltenberg, who will have a working lunch with EU leaders Thursday, pointed to the "cracks and divisions" in Russia, exposed by Wagner's actions.

But he warned it was "too early to draw any final conclusions".

Stoltenberg added that he expected new announcements of military support to Ukraine during a NATO meeting next month in Vilnius.

Borrell also said Europe needed to support Ukraine for the "long haul", supporting the country "during the war and after" and recommended a "Ukrainian defence fund".

The fund, he said, would support the modernisation of the Ukrainian army, with a commitment from the EU to ensure the war-torn country's security.

Lithuanian President Gitanas Nauseda echoed Borrell's sentiments earlier Thursday, as he called for "decisive" action to confront the Russian threat.

"We have to be aware that any movements in Russia's political system will have repercussions on our security but I don't agree with colleagues that sometimes say a strong Putin is less dangerous than a weak Putin. I don't agree with that," he said.