The Biden administration is exploring options for a potential evacuation of US citizens from Ukraine if Russia were to invade the country and create a dire security situation, half a dozen sources tell CNN. The contingency planning is being led by the Pentagon, the sources said, and comes as the administration briefs Congress on how the US is preparing. In a “gloomy” briefing to senators by senior State Department official Victoria Nuland on Monday night, Nuland outlined the tough sanctions package being prepared by the administration in response to a potential Russian attack, but acknowledged that the US’ options to deter an invasion are fairly limited, a person familiar with the briefing said. It is still unclear whether Russian President Vladimir Putin has made the decision to invade, US officials stressed. But he has amassed enough forces, equipment and supplies near Ukraine’s borders that he could move to attack on very short notice. The administration does not currently see a need for evacuations, the sources stressed —airlines are still operating from Ukraine’s international airports and land borders to Ukraine’s western neighbors are open. The discussions are part of planning in the event the security situation severely deteriorates, multiple officials involved in the planning emphasized. “The Department of Defense is a planning organization and must be ready for any manner of contingencies around the world. We do a lot of thinking about a lot of scenarios. But there is no demand signal for civilian evacuations in Ukraine, and it would be wrong to conclude that there is an active effort in the Pentagon to prepare for them,” Pentagon press secretary John Kirby said in a statement. In a tweet, the US Embassy in Kyiv said, “The U.S. government is not currently considering evacuations of U.S. government personnel or American citizens from Ukraine. We are conducting normal contingency planning, as we always do, in the event the security situation severely deteriorates.” The Defense Department and State Department have plans ready for worst-case scenarios in countries around the world, and plans are updated and changed in light of the situation on the ground. But an evacuation of diplomatic staff from Ukraine remains very unlikely. Even if Ukraine is not a NATO member, the White House sees the country as a key Eastern European ally, and there is a strategic benefit in the presence of US diplomats and training forces in Ukraine. That makes a drawdown of the US footprint there far less likely than, for example, the drawdown of US diplomats and troops in Afghanistan. Kyiv, which is the only city where the US has a diplomatic presence in Ukraine, is hundreds of miles from Crimea or the Ukraine-Russia border, where Moscow has built up forces. Such a move would be a propaganda win for Putin, sending a message that the Russian leader can influence the US’ policies and diplomatic presence in Ukraine. But the deliberations reflect how seriously the administration is taking the potential for an invasion. President Joe Biden is speaking with Putin on Tuesday morning amid the heightened tensions. Right now, the Pentagon is planning for a number of different scenarios in the event that an evacuation is necessary, ranging from a smaller evacuation of just nonessential US government employees to a larger one involving a broader swath of American citizens, the sources said. The State Department would be responsible for ultimately determining whether an evacuation operation is necessary. Right now some planning is underway by the State Department’s diplomatic security for either an authorized or ordered departure of diplomats from the country should the situation warrant it, a source familiar with the discussions said. “We don’t know that Putin has made up his mind to use force, but what we do know is that he’s putting the Russian military, the Russian security forces in a place where they could act in a pretty sweeping way,” CIA Director Bill Burns said at a Wall Street Journal CEO Council Summit on Monday. Administration wants to avoid getting caught ‘flat-footed’ Several sources noted that the flurry of planning comes on the heels of the massive US evacuation from Afghanistan, which lawmakers criticized as poorly planned after images surfaced showing chaos at the Kabul airport. The US does not want to be caught “flat-footed” in the event of an unexpected invasion, said one source familiar with the matter. The military has similar contingency planning in place for US personnel stationed in Ethiopia, where an ongoing conflict between rebel groups and the central government has continued to deteriorate. Still, officials emphasized that any potential evacuation in Ukraine likely would not resemble the massive US effort to evacuate all American citizens from Afghanistan earlier this year. Nuland, the undersecretary of state for political affairs, said Tuesday that a range of 10 to 15,000 Americans are currently in Ukraine – a number that tends to fluctuate around Christmas time, as more people visit. “If we are concerned about imminent hostilities in Ukraine, we will begin warning US citizens as we do all around the world … that it’s time for them to find their way home,” Nuland said in an interview with CNN’s Jake Tapper on “The Lead.” Typically, if a security situation deteriorates, the US Embassy increases its messaging to US citizens to advise them to leave the country on commercial flights, and will offer assistance for those who are not able to afford to do so. The US Embassy in Kyiv has not issued any new advisories urging American citizens to consider leaving Ukraine. The last security alert was issued in late November, advising US citizens “of concerning reports of unusual Russian military activity near Ukraine’s borders and in occupied Crimea.” “U.S. citizens are reminded the security conditions along the border may change with little or no notice,” it said. The State Department has long warned US citizens against traveling to Crimea because of the conflict — and the entire country is currently at a Level 4: Do Not Travel advisory due to the Covid-19 pandemic. Some sources cautioned that any evacuation effort from Ukraine — if done under military pressure from Russia — would be a massively complex logistical challenge. A recent US intelligence assessment found that Russia could be preparing to invade as early as next month with as many as 175,000 troops from positions near Ukraine’s southern, western, and northeastern borders. On Capitol Hill, and in some corners of the Pentagon, pressure is building for the United States to provide new Javelin anti-tank and anti-armor missiles to Ukraine — something that almost certainly would not make a strategic difference to Ukraine’s ability to fend off a full-scale invasion by Russia, but might buy some time. NATO allies “have met and will continue to meet to determine from the full range of political and military tools available what they wish to do,” a senior western intelligence official told CNN in a recent interview. The US is also taking steps to plan for additional US troop deployments in Europe on NATO’s eastern flank to help reassure allies of continued American support in the event of an invasion, a senior administration official said on Monday. “I think you could anticipate that in the event of an invasion, the need to reinforce the confidence and reassurances of our NATO allies and our eastern flank allies would be real. And the United States would be prepared to provide that reassurance,” the official said. This story has been updated with additional details Tuesday.