MOSCOW (Reuters) -Russia will start deploying tactical nuclear weapons in Belarus after special storage facilities are ready on July 7-8, President Vladimir Putin said on Friday, Moscow's first move of such bombs outside Russia since the fall of the Soviet Union.

More than 15 months into the biggest land war in Europe since World War Two, Putin says the United States and its Western allies are pumping arms into Ukraine as part of an expanding proxy war aimed at bringing Russia to its knees.

Putin in March announced he wanted to deploy tactical nuclear weapons in Belarus, an apparent warning to the U.S.-led NATO military alliance over it support for Ukraine.

Leading NATO countries say they will support Ukraine and help it defend itself for as long as it takes from what Kyiv casts as an imperial-style land grab by Russia which threatens the survival of the Ukrainian state.

"Everything is going according to plan," Putin told Belarusian President Alexander Lukashenko when discussing the planned nuclear deployment during talks at the Russian leader's Black Sea summer retreat in Sochi.

"Preparation of the relevant facilities ends on July 7-8, and we will immediately begin activities related to the deployment of appropriate types of weapons on your territory," Putin said, according to a Kremlin transcript of his remarks.

Lukashenko said: "Thank you, Vladimir Vladimirovich."

Putin, 70, casts the war as a battle for Russia's own survival in the face of what he says is an ever-expanding NATO. He has warned the West that Moscow will not back down.

Ukrainian President Volodymyr Zelenskiy says Ukraine will not rest however until every last Russian soldier is ejected from his country, which he wants to join NATO as soon as possible.

Putin's nuclear move is being watched closely by both the United States and its NATO allies in Europe and by China, which has repeatedly cautioned against the use of nuclear weapons in the conflict.

After the 1991 fall of the Soviet Union, the United States and Russia worked hard to return the vast Soviet nuclear arsenal, then also deployed in newly independent republics such as Ukraine, Kazakhstan and Belarus, to Russia.

Only the United States had so far used nuclear weapons in anger - in the 1945 attacks on the Japanese cities of Hiroshima and Nagasaki.

(Reporting by Guy Faulconbridge; Editing by Kevin Liffey/Andrew Osborn)