U.S. and Russia prepare to confront each other at the U.N. over Ukraine.

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 former Soviet republic that has become a catalyst for the lowest point in U.S.-Russian relations 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, “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.”

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 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.