President Vladimir Putin said on Wednesday that a military strike against North Korea designed to destroy its nuclear and missile program might not succeed because Pyongyang could have hidden military facilities that nobody knows about.

Russia is strongly opposed to the idea of such a strike, an idea U.S. President Donald Trump has floated, favoring a mixture of diplomacy and economic incentives instead.

But Putin, speaking at an energy forum in Moscow, mused on the subject on Wednesday, making it clear he had serious doubts about the military efficacy of such a move, as well as other political and moral concerns.

“Can a global strike against North Korea be launched to disarm it? Yes. Will it achieve its aim? We don’t know. Who knows what they have there and where. Nobody knows with 100 percent certainty as it’s a closed country.”

Putin said Russia had more reason than most to be concerned by Pyongyang’s missile program, saying that North Korea’s nuclear testing range was located just 200 kilometers (124.27 miles) from the Russian border.

The Russian leader also reiterated his call for diplomacy to be allowed to run its course and for all sides to dial down the bellicose rhetoric. He also said he thought Trump was listening to Russia’s views on the crisis.

More sanctions were the road to nowhere, Putin told the same forum, saying around 40,000 North Korean citizens were currently working in Russia.

Such workers are known to regularly send back part of their wages to the North Korean authorities.