As Russian President Vladimir Putin masses troops at the border of Ukraine, now numbering about 100,000, Vice President Kamala Harris says that the U.S. is ready to impose punishing sanctions if Russia invades. "We are very clear that Russia should not invade the sovereignty of Ukraine, that we must stand up – and we are standing up – for its territorial integrity," she told CBS News chief foreign affairs correspondent and "Face the Nation" moderator Margaret Brennan. "We are working with our allies in that regard, and we've been very clear that we are prepared to issue sanctions like you've not seen before." The vice president, who spoke with Brennan in an interview Monday, declined to say whether these new sanctions would target Putin directly. "I'm not going to talk about specific sanctions, but we are making that clear to him, and we are in direct conversations." Despite the large Russian military buildup at the border, the U.S. does not yet think that Putin has made a decision about invading Ukraine. The international community's concerns about a possible invasion stem from Putin's 2014 annexation of Crimea, which is viewed as a violation of Ukraine's sovereignty and not recognized by the international community. Putin opposes NATO expansion eastward into his sphere of influence, and he does not want Ukraine to obtain NATO membership. Ukraine has embraced Europe and very much wants to join the NATO alliance. There is currently a level 4 State Department "do not travel" travel advisory in place for Ukraine. The reason not to travel given in an earlier advisory was COVID-19, but the newest advisory, issued Monday, cites the military threat posed by Russia. "U.S. citizens should be aware of reports that Russia is planning for significant military action against Ukraine," the advisory reads. This is the department's highest advisory, which warns of a "greater likelihood of life-threatening risks" and an environment that could make it extremely difficult for the U.S. government to aid any traveler in distress during an emergency. The Biden administration sanctioned Russia in April for a different reason – to retaliate for incursions including the SolarWinds cyber espionage campaign and Moscow's interference in the 2020 presidential election.