The U.S. warned of "catastrophic consequences" if Moscow used nuclear weapons in Ukraine on Sunday (Sep 25), after Russia's foreign minister stated territories hosting highly criticized referendums would be fully protected if seized by Moscow.

On Sunday, votes in four eastern Ukrainian districts aiming at annexing land gained by force by Russia were held for the third day. Within days, the Russian parliament could vote to formalize the annexation.

By merging Luhansk, Donetsk, Kherson, and Zaporizhzhia into Russia, Moscow might depict efforts to recover them as attacks on Russia itself, a message to Kyiv and its Western backers. According to US National Security Advisor Jake Sullivan, the US has warned Moscow of the "catastrophic repercussions" it would experience if it used nuclear weapons against Ukraine. The US has also threatened to respond to any such use.

Sullivan said on NBC's Meet the Press television show that "If Russia crosses this line, there will be catastrophic consequences for Russia. The United States will respond decisively,"

The most recent US warning came after President Vladimir Putin on Wednesday issued a barely veiled nuclear threat, saying that Russia will use all means necessary to defend its borders.

The point was more explicitly made by Foreign Minister Sergei Lavrov at a news conference on Saturday following his address to the UN General Assembly in New York, in which he repeated Moscow's fabrications used to justify the invasion, namely that the legitimately elected government in Kyiv had been overthrown and replaced with neo-Nazis.

When asked if there would be justification for Russia to use nuclear weapons to defend annexations, Lavrov responded that all the Russian territory, including those that may one day be "further enshrined" in the Russian constitution, "is under the full protection of the state".

Japan announced on Monday that it had decided to forbid the transfer of goods connected to chemical weapons to Russia. According to a government announcement on Monday, it expanded the list of 21 Russian organizations that were subject to the export bans already in place, including science labs.

Chief Cabinet Secretary Hirokazu Matsuno stated in a media briefing that "Japan is deeply concerned about the possibility of nuclear weapons used during Russia's invasion of Ukraine." Adding that Japan will continue to cooperate with the international community in support of Ukraine and sanctions against Russia.

The referendums have been rejected by Ukraine and its supporters as a ruse orchestrated by Moscow to justify escalating the conflict and launching a mobilization effort in response to recent fighting losses.