UK Prime Minister Boris Johnson said Friday there may be no swift end to the conflict in Ukraine because of the stiff resistance to Russia's invasion.

Asked if he agreed with defence intelligence assessments that fighting could drag on until the end of next year, he told reporters: "The sad thing is that is a realistic possibility."

Johnson, on a two-day visit to India to boost bilateral ties, said Russia's Vladimir Putin had made a "catastrophic blunder" in ordering the invasion.

"The only option he now has really is to continue to try to use his appalling grinding approach," he added.

But he paid tribute to the Ukrainian resistance, and outlined planned and potential military support for the country and its neighbours.

Johnson said a "realistic" negotiation to end the conflict "doesn't seem likely at the present time" but Europe and Ukraine itself would eventually need to discuss future security arrangements.

"What the Ukrainians want, and I think are now going to get, is a collection of security guarantees from like-minded countries, security commitments about what we can do to back them up," he added.

That could include weaponry, training and intelligence sharing to potentially allow Ukraine to prevent future Russian attacks.