Pope Francis’ view on nuclear weapons can be summed up in a few simple words.

Just say no.

The use and possession of weapons should be made “immoral” under official Catholic teaching, Francis told reporters aboard the papal plane Tuesday taking a new step in terms of the Vatican’s position on this issue.

He also expressed reservation concerning the use of nuclear energy.

“An accident (through the) possession or the folly of some leader could destroy humanity,” Francis said. “I have a personal opinion: I wouldn’t use nuclear energy until it is totally safe to use it.”

Francis made the comments during a news conference on his way back from a weeklong apostolic visit to Thailand and Japan.

He also commented on the political unrest in his native South America, which he admitted to be “in flames,” and the ongoing protests in Hong Kong against the extradition of its nationals to China.