Held on Thursday alongside the annual UN General Assembly gathering of world leaders, the session followed a striking development in the war this week: Russia called up a portion of its reserves for the first time since World War II.

At the same time, President Vladimir Putin said his nuclear-armed country would "use all means available to us" to defend itself if its territory is threatened.

US Secretary of State Antony Blinken saw Putin's remark as particularly menacing given plans for referendums in Russian-controlled parts of eastern and southern Ukraine on whether to become part of Russia.

Western nations have condemned those votes as illegitimate and nonbinding. But, in their wake, Moscow might see any Ukrainian attempt to retake those areas as an attack on "Russian territory," Blinken warned.

"Every council member should send a clear message that these reckless nuclear threats must stop immediately," he said.

Russian Foreign Minister Sergey Lavrov didn't mention his country's nuclear capacity or the new troop mobilisation during his own remarks at the council meeting, which France called to discuss accountability for alleged abuses and atrocities during the nearly seven-month-long war.

Instead, Lavrov repeated his country's frequent claims that Kyiv has long oppressed Russian speakers in Ukraine's east -- one of the explanations Moscow has offered for the invasion -- and that Western support for Ukraine is a menace to Russia.

"What's particularly cynical is the position of states that are pumping Ukraine full of weapons and training their soldiers," he said, maintaining that their goal is to prolong fighting "to wear down and weaken Russia".

"That policy means the direct involvement of the West in the conflict," Lavrov said. He added that Ukraine had become "an anti-Russia staging ground to create threats against Russian security" and his country wouldn't accept it.

The Security Council has held dozens of contentious meetings on Ukraine since the war began in February, but Thursday's session had special stature.

"That President Putin picked this week, as most of the world gathers at the United Nations, to add fuel to the fire he started shows his utter contempt and disdain for the UN Charter, the UN General Assembly and this council," Blinken told foreign ministers around the group's famous horseshoe-shaped table.

"Tell President Putin to stop the horror he started. Tell him to stop putting his interests above the interests of the rest of the world, including his own people," Blinken added.

Regardless, no one expects the council to act against Russia, since Moscow has veto power as a permanent member.

But the meeting was still a rare moment for top diplomats from Ukraine and Russia to appear in the same room -- made all the more extraordinary for the fact that Lavrov is under US sanctions.

Blinken argued that Russia should face further censure and isolation for its invasion, pressing other countries to join in Washington's forceful condemnations of the conflict. He cited the discovery of mass graves in Ukraine and repeated allegations from Ukrainians that they were tortured by Russian soldiers.

Australian Associated Press