“How can you deal with a crocodile when it’s in the middle of eating your left leg?”

That was U.K. Prime Minister Boris Johnson, stressing the difficulty of Ukraine trying to negotiate with Russia, which invaded its neighbor nearly four months ago. President Vladimir Putin can’t be trusted, and Ukraine needs military help urgently, he said.

“What’s the negotiation? That is what Putin is doing. He will try to freeze the conflict, he will try to call for a cease fire, while he remains in possession of substantial parts of Ukraine,” said Johnson in an interview with Bloomberg that aired Friday.

Russian-backed separatists on Friday continued to push into strongholds in Ukraine’s eastern territory, even as the invader’s offensive in the Donbas region appeared to slow. Ukraine’s foreign minister warned Thursday that the country’s forces would struggle to prevent Russia’s advance on the east without more “weapons, weapons and weapons again.”

Despite valor on the part of Ukraine’s forces and President Vlodomir Volodymyr Zelensky, the country is facing a massive undertaking, said Johnson. “I’m afraid that Putin, at great cost to himself and to Russia…is continuing to make gradual, slow, but I’m afraid palpable progress and it’s absolutely vital that we continue to support the Ukrainians militarily.”

Johnson said the West needs to expedite sending longer-range Multiple Launch Rocket Systems (MLRS), something that Ukraine has been pushing for, though some worry that could drive NATO and western nations closer to a direct confrontation with Russia.

Johnson said the world “desperately” need the conflict to end. And the “one way it can end is for Putin to accept that the denazification of Ukraine has taken place, and that he’s able to withdraw with dignity and honor,” said Johnson, in an interview with Bloomberg.

Earlier this month, Dmitry Konstantinovich Kiselyov, a Russian television presenter, news executive and propagandist, said Putin had threatened to obliterate the U.K. with his most powerful nuclear weapon that would trigger a 1,600 foot radioactive tidal wave. That’s amid the country’s ongoing support of Ukraine and tensions over Finland and Sweden’s aims to join NATO.