Russia and China have completed their weeklong joint naval exercises in the East China Sea, Reuters reported.

The exercises, which began last week, included drills to practice capturing an enemy submarine, according to the news service.

“The ships of the two countries, with the support of anti-submarine aviation, jointly searched for a submarine of a conditional enemy and fired a volley of jet depth charges,” Russia’s defense ministry said, according to Reuters.

The initiative marks the latest strengthening of military cooperation between Moscow and Beijing since Russia invaded Ukraine in late February.

The invasion triggered strict sanctions and international condemnation from the West, but China has so far refused to criticize Russia for the war. 

Moscow has also signaled support for Beijing as tensions grow between China and the U.S. over Taiwan.

Russia and China have engaged in multiple military drills since the invasion, including nuclear-capable bomber flights in May that took place during President Biden’s visit to Japan.

A joint exercise in September involved more than 2,000 Chinese troops, and the two countries in November flew bombers in joint patrols over the Sea of Japan and the East China Sea.