Shortly before dawn and minutes after Russian President Vladimir Putin announced a “military operation” in Ukraine early Thursday, Russia launched attacks on major cities and airports across Ukraine, shelling more than a dozen cities and towns and crossing the border in multiple locations.

On the first day of the invasion, Ukraine’s Ministry of Internal Affairs reported nearly 400 instances of shelling by Russian forces and 100 Russian-launched missiles fired from land and sea. Multiple explosions were reported around Ukraine, including at Melitopol Air Base and several other airports. Ukrainian officials said more than 137 soldiers and civilians had been killed, and as many as 316 people had been injured since the invasion began. The blasts in Kyiv on Friday have already displaced at least 100,000 people. Defense officials believe the attacks are part of an “initial phase” of a large-scale Russian invasion.

As missiles continue to rain down on Kyiv, Ukrainians and foreigners rush to evacuate the country. Trains carrying children evacuated from eastern Ukraine. The Organization for Security and Cooperation in Europe also “temporarily evacuate[d]” all its international mission members from Ukraine. Haaretz reported that the Jewish community of Odessa has begun evacuating members to nearby countries. And, pictures and videos on social media show long lines at several bus and train stations, major roads with severe traffic jams, and thousands of people pouring into neighboring countries.

The U.S. has said it will not send troops to rescue Americans currently in Ukraine out of security concerns that such military action could escalate to an all-out war. Furthermore, the Overseas Security Advisory Council (OSAC) has recommended that U.S. citizens depart Ukraine immediately if safe to do so, using commercial or privately available ground transportation options.

In addition, the European Union Aviation Safety Agency warned that the presence and possible use of a wide range of ground and airborne warfare systems pose a high risk for civil flights operating at all altitudes and flight levels.

And, with Ukrainian airspace closed to all civilian flights, international security expert Dale Buckner, says, “It’s become much harder to leave.” Buckner, CEO of Global Guardian, has been helping to evacuate people, prepare plans, brief families and employees of corporations with business in Ukraine, covering safety and security of 400 expatriates and 3,000 local nationals. “With bombs falling, this is a true humanitarian crisis,” he says.

In the meantime, Buckner suggests that governments and corporations operating in the country must have a strong communications plan and assets in place, such as satellite phones, in the event that a wide-scale Russian cyberattack shuts down communications in Ukraine.

Those who remain in Ukraine may soon find themselves cut off from the outside world, he believes. “We should anticipate those lines of communication starting to deteriorate very quickly over the next 24-48 hours.” While the situation is fluid and unclear, entities should determine which employees are deemed essential in Ukraine and cannot leave, and which employees can safely evacuate the country.

In addition, Buckner recommends that as the situation in Ukraine continues to evolve, entities must continue to assess the risk and threat level impacting their people on the ground. “Will this be just a true occupation of the East and in the West of Ukraine? Will there only be bombings? Will there be a takeover? All of these questions are on the table and difficult to answer,” he says.

Organizations looking to safely evacuate assets in Ukraine should consider using ground transportation provided by Ukrainians. However, it’s important to note that those capabilities are severely limited. Buckner predicts transportation will only be available for the next 7-10 days. Recently, Ukraine’s National Security and Defense Council Chief Oleksiy Danilov called for a national state of emergency, subject to parliamentary approval, which could impact those looking to evacuate and increase security at public facilities, traffic restrictions and additional transport and document checks.

Therefore, Buckner says, moving quickly is essential as the borders of Poland, Romania and other neighboring countries will become overwhelmed. Buckner adds, “In a few weeks, Russia will likely open up air corridors for relief flights to evacuate Western expatriates wishing to leave Ukraine because it is not in Russia’s best interest to have European or U.S. expatriates harmed in that way.”

U.S. citizens are encouraged to ensure travel documents are valid and easily accessible, review personal security plans and have a contingency plan that does not rely on U.S. government assistance. Those seeking emergency assistance and those who decide to remain in Ukraine should complete this online form, and the State Department will respond. U.S. citizens seeking to depart Ukraine can also call 1-833-741-2777 (in the United States) or 1-606-260-4379 (from overseas) for immediate assistance.