President Biden's decision to allow Ukraine to use US-supplied long-range missiles against targets in Russia has provoked strong reactions from Russian officials, with warnings of serious escalation and potential consequences. Russian leaders, including MP Leonid Slutsky and senator Vladimir Dzhabarov, have criticized the move, while Putin's spokesman Dmitry Peskov indicated it could lead to increased tensions. The Kremlin has previously warned against such actions, hinting at possible changes to Russia's nuclear doctrine. The situation is complicated by the upcoming transition to President-elect Trump, who may have different views on military support for Ukraine.