Former U.S. Ambassador to the United Nations John Bolton said Thursday there needs to be a “very strong response” to Russia’s assassination attempt against ex-Russian spy.

Although he was pleased with May’s decision to expel 23 Russian diplomats from the U.K., Bolton told Fox News he doesn’t think it goes far enough in motivating Russia to stop such attacks.

“We need a very strong response to this. This is unacceptable conduct, using a weapon of mass destruction, and there are capitals all over the world... that are watching what we do,” Bolton said.

He also compared Russian President Vladimir Putin’s actions to North Korean leader Kim Jong Un killing his brother by having women rub VX nerve agent on his face at an airport in Malaysia.

“It’s an act of defiance, it’s saying to London and to the other western capitals, what are you going to do about it? Well I think there should be a very strong answer to that,” Bolton reiterated.

“I think what Prime Minister May did was the right thing, I would not call it a strong response. I think the response needs to be as such that we begin to create, in Vladimir Putin’s mind, deterrence theories. That he will understand if he undertakes this again the cost that Moscow will bear will be significantly greater. That’s how deterrence works.,” Bolton said.

