Russia’s defense chief on Tuesday signed a five year plan for closer military ties with China, pointing to increasingly frequent US strategic bomber flights near both countries’ borders.

During a video call, Defense Minister Sergei Shoigu and his Chinese counterpart Wei Fenghe expressed a shared interest in stepping up strategic military exercises and joint patrols by Russia and China, according to the Russian Defense Ministry.

China and Russia have been strategic partners for many years, Shoigu said.

He pointed to increasingly intensive flights by the U.S. strategic bombers near Russian borders, saying that there were 30 such missions over the past month alone.

Russia has sought to expand ties with China as its relations with the U.S. and its allies sank to post-Cold War lows over Moscow’s 2014 annexation of Ukraine’s Crimea Peninsula, accusations of Russian hacking attacks, interference in elections and other disputes.