Russian President Vladimir Putin will hold a signing ceremony in the Kremlin on Friday to formally annex the areas of Ukraine that Russia has occupied, his spokesman has said.

The ceremony will be to sign “treaties of accession” with the four regions created by Russia’s occupation forces -- the two self-declared Donetsk and Luhansk People’s Republics and the Zaporizhzhia and Kherson regions.

Putin will also deliver a major speech to lawmakers gathered there, his spokesman said.

It is a major moment in the war -- another no-going-back moment for Putin. In reality, none of the areas being annexed are under full control of Russia right now as all are seeing fighting and facing Ukrainian efforts to re-take them.

If Putin attempts to annex the occupied regions, it will be one of the most egregious violations of international law in Europe since World War II.

-ABC News' Patrick Reevell

American citizens are being advised by the U.S. State Department to get out of Russia immediately.

The U.S. Embassy in Moscow has issued an alert, saying "severe limitations" could prevent it from assisting U.S. citizens still in the country.

"If you wish to depart Russia, you should make independent arrangements as soon as possible," the alert said.

Noting that Russia has begun a military mobilization against Ukraine, U.S. Embassy officials warned Americans with dual Russian citizenship that they could get drafted by Russia.

"Russia may refuse to acknowledge dual nationals U.S. citizenship, deny their access to U.S. consular assistance, prevent their departure from Russia, and conscript dual nationals for military service," the alert said.

The alert also advised U.S. citizens to avoid political or social protests in Russia, saying Americans have been arrested in Russia for participating in demonstrations.

"We remind U.S. citizens that the right to peaceful assembly and freedom of expression are not guaranteed in Russia," the alert said.

Roughly 66,000 Russian citizens have fled across borders into European countries amid Russian President Vladimir Putin's announcement last week of a military mobilization against Ukraine, the European Border and Coast Guard said Tuesday.

The number of Russian citizens pouring into Europe was up 30% compared to last week, according to the agency which also goes by the name Frontex.

Most of the Russian citizens are entering the European Union through Finnish and Estonian border crossing points, Frontex said on Twitter.

Putin announced on Sept. 21 that he is ordering the mobilization of 300,000 recruits to fight in Ukraine, prompting widespread protests and clashes with police across Russia.

In recent days, photos have emerged of huge traffic jams at border crossings. On Monday, the wait at the border between Russia and Georgia was estimated to be 40 to 50 hours, according to the independent Russian news outlet The Insider.

Partial results from what Ukraine and its Western allies have called "sham" referendums in four Russian-occupied regions of Ukraine show that more than 96% of voters favor becoming part of Russia, according to the state-owned Russian news agency RIA.

Voting has taken place over five days in the four areas -- Donetsk, Luhansk, Zaporizhzhia and Kherson.

The early results showed that 97.93% of voters in the Luhansk People's Republic favored joining the Russian Federation, according to the data. In Donetsk People's Republic, early results showed 98.69% favored joining the Russian Federation.

In Zaporizhzhia, 97.81% of voters cast ballots to join Russia and 96.75% of voters in Kherson also favored joining Russia, according to the data.

President Joe Biden and other Group of 7 leaders condemned Russia's "sham referenda" in occupied Ukrainian territories, calling it a Russian attempt to "create a phony pretext for changing the status of Ukrainian sovereign territory."