Top South Korean, U.S. and Japanese military officers urged North Korea to cease all unlawful activities that threaten regional security, as the three nations flew advanced warplanes for a joint exercise in a show of force against the North, Associated Press reported.

The development came Friday as Russian Foreign Minister Sergei Lavrov was to travel to North Korea amid booming military and other cooperation between the two countries that have raised concerns among their neighbors. 

During their regular meeting in Seoul on Friday, the chairmen of the joint chiefs of staff from South Korea, the U.S. and Japan discussed North Korea’s deployment of troops to Russia to support its war against Ukraine and Russia’s potential transfer of military technology to North Korea in return, according to Associated Press.

“They urged the DPRK to immediately cease all unlawful activities to destabilize the Korean Peninsula, the Indo-Pacific, and beyond, and pledged to continue working together to respond to the DPRK’s threats,” the three top military officers said in a joint statement.