Ukraine is on the precipice of a daunting calamity following "dangerous and disturbing" developments, UN Secretary-General Antonio Guterres warned Thursday after Russia again raised the specter of nuclear war. 

Guterres told the Security Council that Ukraine is increasingly headed "towards an endless cycle of horror and bloodshed."

"As I have said from the start, this senseless war has unlimited potential to do terrible harm – in Ukraine, and around the world. The idea of nuclear conflict, once unthinkable, has become a subject of debate," he said. "This in itself is totally unacceptable. All nuclear-armed states should recommit to the non-use and progressive elimination of nuclear weapons."

Russian President Vladimir Putin announced Wednesday a mass mobilization in Russia of 300,000 reservists to support his flagging war effort in Ukraine. In doing so, he offered a dire warning ahead of planned referenda in parts of Ukraine, which Putin and his proxies are hoping to use to claim additional Ukrainian territory.

Putin said Moscow would not hesitate to use all of the weapons in its arsenals to protect "the territorial integrity of our country."

"When the territorial integrity of our country is threatened, we will certainly use all the means at our disposal to protect Russia and our people. This is not a bluff," he said.

Guterres voiced deep concern at the prospects of the referenda, emphasizing that "any annexation of a state’s territory by another state resulting from the threat or use of force is a violation of the UN Charter and of international law."

"I appeal to all member states, and especially those here today, to redouble all efforts to prevent further escalation, and to do all they can to end the war and to ensure lasting peace," he said.

Also addressing the council, Secretary of State Antony Blinken said the international order "is being shredded before our eyes."

"We cannot, we will not allow President Putin to get away with it," he said. "Defending Ukraine's sovereignty and territorial integrity is about much more than standing up for one nation's right to choose its own path, fundamental as that right is."​​​​​​​​​​​​​​