"The prospect of nuclear conflict, once unthinkable, is now back within the realm of possibility," says Antonio Guterres, Secretary-General of the United Nations. 

The increased risk of nuclear war is because of Russia's invasion of Ukraine. Now, Russia has used a hypersonic missile. It can travel at up to 7,000mph and hit targets 1,200 miles away. The missile can also carry nuclear warheads. 

Russia and the USA have enough nuclear weapons to kill hundreds of thousands of people. Nuclear war would have terrible long-term effects like famine and environmental disaster.

Most experts do not believe that Putin will cause a war like this. "His objective is not to bring the world to nuclear war," says Paul Hare, an academic and diplomat. He knows that nuclear war would be as bad for him as his enemies.

But he might use a single, smaller nuclear weapon. Reports suggest that he is prepared to do this if he feels threatened.

Even these terrible consequences. It would be the first time that a nuclear weapon has been used since 1946.

If Russia were to drop a nuclear bomb, would the USA use one too? Nobody knows. The American government has set up war games to work out how the situation would play out. One ended with them retaliating, the other did not.

Most experts think the possibility of nuclear war is still small. But it is not impossible. "The chance for nuclear weapons employment is extremely low," says Ulrich Kuhn of Hamburg University. "But it's not zero. It's real, and it might even increase."

Yes: Putin clearly has no concern for human life. Now he is isolated and humiliated. He could do anything. Pressing the nuclear button is a possibility. 

No: Putin is rash and aggressive, but not insane. He is not prepared to sacrifice Russia or himself. He knows nobody can win in a nuclear war.

Or... The main threat is not the temperament of one individual but the fact that nuclear weapons exist at all. As long as people make and keep these terrible weapons, massive destruction is possible. We must get rid of them.