The U.S. official said that Russia had compiled lists of Ukrainians to target.
In a letter to the U.N.'s Human Rights chief, the US Ambassador said the U.S. had credible information. The contents of the letter were first reported by The Washington Post.
She said that there was credible information that Russian forces would likely use lethal measures to break up peaceful protests or counter peaceful exercises of perceived resistance from civilians.
The Kremlin denied Crocker's accusations that Russia had drawn up such lists.
The Defense Minister of the Ukranian said on Monday that an invasion was unlikely to happen tomorrow or the day after.
He said that these groups could be set up in a few weeks.
The White House and British Prime Minister Boris Johnson warned that Russia was planning to launch a full-scale attack on Ukraine.
The US estimated last week that there were between 169,000 and 190,000 Russian troops in and near Ukraine, up from 100,000 at the end of January. The updated figures came despite Russian claims that it had begun to withdraw troops from the border.
Russia has denied any plans to invade Ukraine. It wants the organization to scale back its presence in eastern Europe and demands that it never be allowed to join NATO.
According to a report on Monday, Putin is set to address Russia's security council imminently.
According to Peskov, Russian diplomatic officials were active while tensions were growing over Ukraine.
Over the past week, fighting between Ukrainian authorities and pro-Russian rebels in eastern Ukraine has increased. Russian state media and Ukrainian authorities pointed the finger at each other for their actions.
On Monday, the Ukrainian Defense Ministry said Russian occupation forces had carried out another provocation with the aim of accusing Ukrainian service members.
The ministry said in a statement that the aggressor opened fire on Luhansk.
Ukrainian authorities said they had recorded 80 ceasefire violations over the past day, 72 of which involved the use of weapons that were not allowed under the agreements with Russia.
With Ukrainian defenders refraining from any aggressive acts that could possibly spark a violent response, the occupation forces continue to destroy civilian infrastructure.
More than 1,500 ceasefire violations were recorded by the Organization for Security and Co-operation in Europe on Saturday.
Near the Russian border, the east of Ukraine has been the scene of low-level fighting. The number of people who have died in the conflict in eastern Ukraine has risen sharply in the last few days, but the number of violations of the cease-fire has remained the same.
The leader of one of the rebel-held areas in eastern Ukraine said last week that residents would be evacuated to Russia with the help of the Russian government. It came after the country's lawmakers asked Putin to officially recognize two self-declared republics in Ukraine.
The actions of the armed forces of Ukraine had led to Russia taking action to ensure security and save the lives of the civilians, according to the Kremlin.
Russia's state media continued to report on Monday that Ukrainian government forces had attacked the area.
Western officials have warned that Russia could make false claims about the conflict in Ukraine's east to justify military aggression and an invasion.
The Secretary of State told the UN Security Council last week that there could be a fake terrorist bombing, a fake mass grave, or a fake chemical weapon attack.
The White House said on Sunday that President Joe Biden has agreed to a meeting with Putin if there is no invasion of Ukraine. Moscow is preparing for a full-scale assault on Ukraine very soon, the U.S. warned.
There are no plans for a meeting between Putin and Biden according to the Kremlin.
If Moscow does not initiate any military action, the meeting will be held in Europe later this week.
The National Security Council was convened by Biden on Sunday to discuss the situation in Ukraine.
There was a real possibility of war in Europe for the first time in 70 years, according to the Vice President.
She said that the President believes that Putin has made his decision.
Some of the greatest sanctions, if not the strongest, have ever been issued by the U.S. and its allies in the event of an invasion of Russia.