Russia-Ukraine updates: 2 US veterans who joined Ukrainian forces missing

Prior to meeting Russian President Vladimir Putin on Tuesday, U.N. Secretary-General Antonio Guterres issued a statement calling for humanitarian corridors in Ukraine that are "truly safe."

Guterres later raised the issue with Putin during a face-to-face meeting, stressing the urgent need for the creation of safe and effective humanitarian corridors in the war-ravaged Ukrainian port city of Mariupol, where he said thousands of civilians remain trapped, according to the Russian state-run TASS news service. Guterres also proposed the creation of a humanitarian contact group.

"We urgently need humanitarian corridors that are truly safe and effective, and that are respected by all to evacuate civilians and deliver much needed assistance," Guterres said prior to meeting with Putin. "To that end, I have proposed the establishment of a humanitarian contact group, bringing together the Russian Federation, Ukraine, and the U.N. to look for opportunities for the opening of safe corridors, with local cessation of hostilities and to guarantee they are actually effective."

Guterres made his statement following a meeting with Russian Foreign Minister Sergey Lavrov.

While meeting with Guterres, Putin said the U.N. chief has been misled and insisted that humanitarian corridors in Mariupol are functioning, according to TASS.

"You say that Russia's humanitarian corridors are not operating. Mr. Secretary-General, you have been misled: these corridors are operating," Putin said, according to TASS.

Putin told Guterres that up to 140,000 people had fled Mariupol with the assistance of Russia.

"And they can go anywhere. Some want to go to Russia; some want to go to Ukraine. Anywhere! We do not keep them, we provide all kinds of help and support," Putin said, according to TASS.

However, Putin “agreed, in principle, to the involvement of the United Nations and the International Committee for the Red Cross in the evacuation of civilians from the Azovstal plant in Mariupol," according to a readout of the meeting provided by the U.N.

The U.N. General Assembly -- where all 193 countries have a vote -- has unanimously adopted a resolution that creates a new accountability mechanism.

Now, whenever a permanent member of the U.N. Security Council uses its veto power to block a resolution, it will automatically trigger a debate in the General Assembly within 10 days.

The move was made primarily in response to Russia's veto power, which the country has used repeatedly to sink resolutions about its own aggression. It has paralyzed the ability of the Security Council, the United Nation's most powerful body, to check Russia.

The United States, Russia, China, France and the United Kingdom are the five veto-wielding permanent members of the Security Council, while the ten other seats rotate and are won by election.

The United States and Liechtenstein co-sponsored the resolution, with the tiny European country tweeting, "Together we have made sure today that a veto is no longer the last word on issues of peace and security."

-ABC News' Conor Finnegan

U.S. Defense Secretary Lloyd Austin said Tuesday that any public statements by world leaders that the conflict in Ukraine could spin out of control and lead to a nuclear confrontation is "unhelpful and dangerous."

Austin's comments during a news conference at Ramstein Air Base in Germany came a day after Russian Foreign Minister Sergei Lavrov warned of a "serious" risk of nuclear war over Ukraine.

"The danger is serious, real. It can’t be underestimated,” Lavrov said in a Russian state TV interview late Monday.

Lavrov said that following the Cuban missile crisis of 1962, the United States and the former Soviet Union developed an understanding of the rules of conduct between the two superpowers and have avoided a nuclear confrontation for 60 years. "Now there are few rules left," Lavrov said.

Austin was asked during Tuesday's news conference whether the conflict in Ukraine could spin out of control and lead to a nuclear confrontation.

"We certainly will do everything within our power and Ukraine will have the same approach, do everything within their power to make sure it doesn't spin out of control. The international community is focused on that as well," Austin said. "There's always a possibility that a number of things can happen. But, you know, again, I think it's unhelpful and dangerous to rattle sabers and speculate about the use of nuclear weapons."

-ABC News' Matt Seyler

U.S. Defense Secretary Lloyd Austin said meetings like the one on Tuesday with more than 40 NATO allies and other partner nations will now occur monthly.

"To ensure that we continue to build on our progress, we're going to extend this forum beyond today," Austin said during a news conference at Ramstein Air Base in Germany.

"I'm proud to announce that today's gathering will become a monthly contact group on Ukraine's self-defense," he said.

The meetings will focus on strengthening Ukraine's military both in the short and long terms, Austin said.

"The contact group will be a vehicle for nations of good will to intensify our efforts and coordinate our assistance and focus on winning today's fight and the struggles to come," Austin said. "The monthly meetings may be in person, virtual, or mixed."

Austin, who visited Ukraine on Sunday with U.S. Secretary of State Antony Blinken, praised Tuesday's meeting with NATO allies, saying, "We're all coming away with a transparent and shared understanding of a challenge that Ukrainians face."

Austin conveyed a sense of urgency for the international community to help the Ukrainians.

"I applaud all of the countries that have risen, and are rising, to this moment," he said. "But we don’t have any time to waste. The briefings today laid out clearly why the coming weeks will be so crucial for Ukraine. So, we’ve got to move at the speed of war."

Austin thanked Germany for hosting Tuesday's meeting and for offering to send Ukraine 50 Cheetah anti-aircraft systems. He also thanked the United Kingdom for its announcement Monday that it would provide Ukraine additional anti-aircraft capabilities.

"We held an important session today with long-term support for Ukraine’s defenses, including what that will take from our defense industrial bases," Austin said. "That means dealing with the tremendous demand that we’re facing for munitions and weapons platforms, and giving our staunch support to Ukraine while also meeting our own requirements, and those of our allies and partners."

-ABC News' Matt Syler