South Korean leaders' security is in "jeopardy” now that they have agreed to host an American missile defense system, a Russian diplomat said Monday.

“We have to take some retaliatory measures,” Russian Ambassador to China Andrei Denisov told reporters, according to TASS, a state-run media outlet.

U.S. officials, after much deliberation with South Korea, deployed a Terminal High-Altitude Area Defense, or THAAD, system to the Korean Peninsula in response to a series of saber-rattling missile launches and nuclear tests by the North Korean regime. But China and Russia worry that the system will degrade the threat posed by their own ballistic missiles.

"Those who house such facilities on their territory, which our Chinese partners and we think pose a threat to our security, essentially put their security in jeopardy,” Denisov said.

Those concerns are unjustified, according to the top U.S. general in South Korea. “That’s the largest full battery deployment anywhere in the world and the second one in this region directed against North Korea and only against North Korea,” Army Gen. Vincent Brooks told the Aspen Security Forum on Saturday. “It’s 100 percent about defending against North Korean missiles.”

Chinese officials worry that the deployment of the missile defense system will move South Korea one step closer to a trilateral alliance with Japan and the United States, and strengthen American-led defenses against possible Chinese attacks in the region. And the THAAD radar could give American forces additional insight into Chinese systems.

“This would mean that all land and air weapons drills would be exposed, and the frequency and quantity of flights as well as the locations of military airports would be known,” researcher Xu Guangyu of China Arms Control and Disarmament Association, a nonprofit group aligned with the Chinese government, told the South China Morning Post last year.

U.S. officials plan to upgrade the THAAD system, even as Secretary of State Mike Pompeo tries to broker a deal to dismantle North Korea’s nuclear weapons program. “We absolutely hope that diplomacy is successful,” Air Force Lt. Gen. Sam Greaves told Bloomberg last month, “but at the same time we must remain vigilant to provide the capability that’s needed.”

Russian President Vladimir Putin protested such deployments, long before Trump began the diplomatic process with Kim, telling Japanese Prime Minister Shinzo Abe that the United States is using the North Korea threat as a "pretext” to expand its military presence in the region.

“Russia believes this to be a disproportionate response,” Foreign Minister Sergey Lavrov said in 2016, while summarizing a meeting between the Russian and Japanese leaders. “Vladimir Putin also talked at length about the U.S. missile defense system, and its willingness to deploy another positioning area in Northeast Asia of what Russia believes to be a global offensive system, supplementing missile-defense bases in Europe, in the Mediterranean and of course in Alaska.”