China’s air force has carried out exercises near the Korean Peninsula, practicing to defend against a “surprise attack” coming over the sea, Chinese state media said.

The exercises came days after North Korea’s sixth, and most powerful, nuclear test fueled global concern that the isolated nation plans more weapons tests, possibly of a long-range missile.

An anti-aircraft defense battalion held the exercises early on Tuesday, near the Bohai Sea, the innermost gulf of the Yellow Sea that separates China from the Korean peninsula, an official military website, www.81.cn, said.

Troops traveled to the site from central China before immediately beginning drills to fend off the “surprise attack” simulating real battle, it said.

“The troops’ rapid response capabilities and actual combat levels have effectively been tested.”

It was the first time certain weapons, which the website did not identify, had been used to shoot down low-altitude targets coming over the sea, www.81.cn said, without elaborating.

The drills “do not target any particular goal or country,” and were part of an annual plan intended to boost the troops’ capability, China’s Defense Ministry said on its website late on Wednesday, in a response to media.

After weeks of rising tension over North Korea’s actions, South Korea and the United States have been discussing the deployment of aircraft carriers and strategic bombers to the Korean Peninsula.

China is deeply suspicious of any U.S.-backed military build-up in the region, and has repeatedly expressed anger at the deployment of a U.S. anti-missile defense system in South Korea.