The Chinese military has reportedly found traces of deadly sarin gas in the air near China’s border with North Korea.

Japan’s Asahi Shimbun quoted a Chinese government official as saying Friday that military analysis showed traces of sarin gas in the air blowing into China from North Korea in November of last year and February of this year.

The paper said the Chinese military is still in the process of determining whether the discovery was the result of tests or an accident and has strengthened its monitoring activities.

Sarin gas is an extremely toxic substance that was developed as a chemical weapon by Germany during World War Two.