The United States and Russia prepared for confrontation Monday at the United Nations Security Council over the Ukraine crisis, with the Americans vowing to make the Russians justify their massing of troops on Ukraine’s borders and Kremlin diplomats dismissing the meeting as farcical theatrics.

The meeting of the 15-nation council, requested by the United States last week, represents the highest-profile arena for the two powers to sway world opinion over Ukraine. The tensions surrounding the former Soviet republic have brought U.S.-Russian relations to their lowest point since the Cold War.

“Our voices are unified in calling for the Russians to explain themselves,” the American ambassador to the United Nations, Linda Thomas-Greenfield, said on ABC’s “This Week” about the meeting at U.N. headquarters in New York.

“We’re going to go in the room prepared to listen to them,” she said in the Sunday interview, “but we’re not going to be distracted by their propaganda, and we’re going to be prepared to respond to any disinformation that they attempt to spread during this meeting.”

As one of the five permanent members of the council — along with Britain, China, France and the United States — Russia has the power to veto any decision by the majority. But it cannot block the meeting itself.

Russian diplomats have ridiculed the meeting as part of a manufactured contretemps over what they call unjustified Western fears, instigated by the United States, that President Vladimir V. Putin of Russia is preparing to invade Ukraine. The Russians have also seized on complaints by Ukraine’s president and others that the Americans are needlessly sowing panic.

Dmitry Polyanskiy, Russia’s deputy permanent representative at the United Nations, appeared to mock Ms. Thomas-Greenfield’s remarks on Sunday in a Twitter post, saying she viewed the Security Council as “a club of worried people with US telling them what to worry about.”

Mr. Putin, who has not spoken publicly about Ukraine since December, maintained his silence.

His spokesman, Dmitri S. Peskov, told reporters on Monday that Mr. Putin would state his views on the situation “as soon as he determines it to be necessary.”

“I can’t give you an exact date,” Mr. Peskov said. Russian officials continued to maintain they were not at fault for the rising tensions, insisting that the United States was fabricating the threat of a Russian invasion of Ukraine.

That was a rare point of common ground with Kyiv, where President Volodymyr Zelensky has also blamed the United States for needlessly sowing “panic” in Ukraine. “To our regret, the American news media has been publishing a great amount of unverified, distorted and deliberately false and provocative information about what is happening in Ukraine and around it in recent months,” Mr. Peskov said. “This is becoming obvious to almost everyone.”

Russia has sent more than 100,000 troops to the Ukrainian border in recent weeks, part of an increasingly aggressive posture by Mr. Putin to protect and enlarge what he sees as Russia’s rightful sphere of influence in Eastern Europe. The Pentagon said on Friday that Russia had amassed enough forces to stage a full-scale invasion of Ukraine at a time of its choosing.

The Kremlin has accused the NATO alliance of threatening Russia and has demanded that it never admit Ukraine as a member. The possibility of a diplomatic solution has remained unclear at best.

The Biden administration has said it wants a peaceful outcome to the crisis but is preparing for the possibility of what American military commanders have said would be a devastating armed conflict in Ukraine. The administration has vowed to respond with crippling economic sanctions on Russia if it invades Ukraine.