MOSCOW, Jan 19 (AFP) - Former Russian President Dmitry Medvedev, an ally of Kremlin chief Vladimir Putin, warned the North Atlantic Treaty Organization (NATO) on Thursday that the defeat of Russia in Ukraine could trigger a nuclear war.

"The defeat of a nuclear power in a conventional war may trigger a nuclear war," Medvedev, who serves as deputy chairman of Putin's powerful security council, said in a post on Telegram.

"Nuclear powers have never lost major conflicts on which their fate depends," said Medvedev, who served as president from 2008 to 2012.

Medvedev said NATO and other defence leaders, due to meet at Ramstein Air Base in Germany on Friday to talk about strategy and support for the West's attempt to defeat Russia in Ukraine, should think about the risks of their policy.

Russia and the United States, by far the largest nuclear powers, hold around 90% of the world's nuclear warheads. Putin is the ultimate decision maker on the use of nuclear weapons.

While NATO has conventional military superiority over Russia, when it comes to nuclear weapons, Russia has nuclear superiority over the alliance in Europe.

Putin casts Russia's "special military operation" in Ukraine as an existential battle with an aggressive and arrogant West, and has said that Russia will use all available means to protect itself and its people against any aggressor.