MOSCOW (AP) — Russian President Vladimir Putin visited Belarus on Friday to sign a treaty offering security guarantees to Moscow's closest ally.

The planned signing of the document follows the publication of a revised version of Russia's nuclear doctrine, which for the first time placed Belarus under the Russian nuclear umbrella amid the tensions between Moscow and the West over the Ukrainian conflict.

Speaking alongside Belarus' President Alexander Lukashenko, Putin emphasized that the new document includes a potential use of Russian tactical nuclear weapons deployed to Belarus in response to an aggression.

“I'm sure that the treaty will ensure the security of Russia and Belarus,” Putin said in televised remarks.

The revamped doctrine that Putin endorsed last month formally lowered the threshold for Russia’s use of its nuclear weapons, a move that follows U.S. President Joe Biden’s decision to let Ukraine strike targets inside Russian territory with American-supplied longer-range missiles.

The doctrine says Moscow could use nuclear weapons “in response to the use of nuclear and other types of weapons of mass destruction” against Russia or its allies, as well as “in the event of aggression” against Russia and Belarus with conventional weapons that threaten “their sovereignty and/or territorial integrity.”

Lukashenko, who has ruled Belarus with an iron hand for more than 30 years and has relied on Russian subsidies and support, allowed Russia to use his country’s territory to send troops into Ukraine in February 2022 and to deploy some of its tactical nuclear weapons in Belarus.