Read also: Give Putin a golden bridge for a graceful exit, he will destroy it - Wallace
NATO is believed to have warned its members, the newspaper said.
Read also: Russian propaganda researcher Peter Pomerantsev analyzes speech of 'evil and humiliated' Putin
The Kremlin has been signaling its readiness for a significant escalation as Russia loses ground on the battlefield. Fears over Putin's earlier hints that he might resort to such tactics heightened yesterday with claims that a train operated by Russia's secretive nuclear division was destined for Ukraine.
Konrad Muzyka, a Poland-based defence analyst, said the train, spotted in central Russia, was linked to the 12th main directorate of the Russian Ministry of Defense and that it was "responsible for nuclear munitions, their storage, maintenance, transport, and issuance to units."
Read also: Pope addresses Putin and Zelenskyy, calls for peace talks
A senior defense source, however, said that a more likely demonstration of Putin's readiness to use nuclear weapons could come in the Black Sea.
Although they believed such a show of force would be more likely, they said it was "not impossible" that Putin could fire a tactical nuclear weapon in Ukraine. If he chose to do so, he faced a significant risk.
Read also: Putin has at least three doubles, says head of Ukrainian military intelligence Budanov
"They could misfire and accidentally hit a Russian city close to the Ukrainian border, such as Belgorod," the source said.
Putin once again threatened the use of nuclear weapons on Sept. 21, saying that "this is not a bluff."
Read also: Putin claims readiness to negotiate with Kyiv amid annexation of occupied territories
The White House said that in case Russia uses nuclear weapons against Ukraine, the response of Washington and its allies will be decisive. The United States warned the Kremlin of "catastrophic consequences."
According to Ukrainian military intelligence chief Kyrylo Budanov, Russia will not launch a nuclear strike in the near future.