Russia-Ukraine updates: US to ban Russian carriers from its airspace

Senior administration officials on Monday provided more specifics on the sanctions against Russia announced over the weekend and emphasized the drastic nature of these steps, saying the "actions represent the most significant actions the U.S. Treasury has taken against an economy of this size, and assets of this size,” noting the Russian central bank is multiple times larger than Iran's or Venezuela’s.

The U.S. has put into effect sanctions on Russia's central bank that keep Moscow from accessing any of their more than $600 billion in reserves in the U.S., or in U.S. dollars in foreign countries. The sanctions also target Russia’s National Wealth Fund and the Ministry of Finance.

Officials said it was clear from the beginning of the invasion that Russian President Vladimir Putin was planning to use central bank assets to mitigate any sanctions.

“Today's announcement that prohibit transactions with the Central Bank of Russia in the national wealth fund will significantly hinder their ability to do that, and inhibit their access to hundreds of billions of dollars in assets from our actions alone, they will not be able to access assets that are either in United States or in US dollars,” officials said.

“What we've done today is not only preventing them from using those dollars in the United States, but preventing them from being able to use those dollars in other places like Europe or Japan to defend their currency and prop up their institutions. And our goal was to make sure that not only would they not have access to dollars, but also not have access to other currencies," officials said.

“Our strategy -- to put it simply -- is to make sure that the Russian economy goes backwards, as long as President Putin decides to go forward with his invasion of Ukraine,” a senior administration official said.

-ABC News' Sarah Kolinovsky, Justin Gomez

After Russian President Vladimir Putin put Russia’s nuclear deterrent forces on a state of heightened alert this weekend, a White House official confirms the U.S. has not changed its own alert level.

"We are assessing President Putin’s directive and at this time see no reason to change our own alert levels," a White House official confirmed to ABC News.

"We think provocative rhetoric regarding nuclear weapons is dangerous, adds to the risk of miscalculation, should be avoided, and we will not indulge in it," the official added.

The official also noted that, as recently as June, when President Joe Biden met Putin face-to-face in Geneva, the two leaders affirmed nuclear war is tantamount to mutually assured destruction.

The leaders said in a joint statement in June, "Today, we reaffirm the principle that a nuclear war cannot be won and must never be fought."

-ABC News' Sarah Kolinovsky

Russia’s central bank on Monday raised its key interest rate to 20% from 9.5% in an apparent effort to slow the fallout from severe international sanctions. 

The rate hike came as the Russian ruble tumbled, trading down as much as 30% against the U.S. dollar on Monday, according to Bloomberg. The currency traded about 17% lower midday in Moscow. 

The Russian stock market reportedly closed for the day. 

-ABC News’ Zunaira Zaki

More than 500,000 people have fled Ukraine since the Russian invasion began on Thursday, the U.N. Refugee Agency said on Monday.

More than half have crossed the border into Poland, the agency said. Filippo Grandi, the U.N. high commissioner for refugees, had said on Sunday that 368,000 people had fled to neighboring countries.

-ABC News’ Zoe Magee