Ukrainian President Volodymyr Zelenskyy said Thursday that the first of 16,000 foreigners who have volunteered to fight for his country have arrived to help battle Russian invaders.

"Ukraine is already greeting foreign volunteers. (The) first 16,000 are already on their way to protect freedom and life for us, and for all," Zelenskyy said in a video shared to his Telegraph page Thursday.

"We have nothing to lose but our own freedom," he added.

According to The Washington Post, foreign citizens have been fighting for Ukraine since 2014, when Russian separatists seized an eastern region of the country. However, The Post says the fighters arriving in Ukraine now are "far beyond" what had previously been seen.

While it's unclear from which countries the volunteers had traveled, some of those arriving in Ukraine may have disobeyed orders from their own government about traveling to the country to fight. The U.S. and the U.K. are among the governments that have instructed citizens against volunteering and instead have provided Ukraine with weapons and aid, CNN reports.

MOSCOW — Presidents Joe Biden and Vladimir Putin have concluded a call to discuss the crisis in Ukraine as the White House says intelligence shows Russia could invade on short notice.

The White House says the call lasted 62 minutes, and that Biden once again called on Putin to de-escalate and pull back the well over 100,000 Russian troops that have massed near Ukraine’s borders.

"President Biden was clear that, if Russia undertakes a further invasion of Ukraine, the United States together with our Allies and partners will respond decisively and impose swift and severe costs on Russia," a White House readout of the call said.

The White House added that "Biden reiterated that a further Russian invasion of Ukraine would produce widespread human suffering and diminish Russia’s standing" and that he was "clear with President Putin that while the United States remains prepared to engage in diplomacy, in full coordination with our Allies and partners, we are equally prepared for other scenarios.

Before talking to Biden, Putin had another call with French President Emmanuel Macron, who met with him in Moscow earlier in the week to try to resolve the crisis.

Russia has massed troops near the Ukraine border and has sent troops to exercises in neighboring Belarus but denies that it intends to launch an offensive against Ukraine.

On Friday, White House officials said they believe that Russia has amassed enough troops on the Ukrainian border to conduct large-scale military operations at "any time."

Previously, analysts believed that Russia would wait to conduct any military action until after the Winter Olympic Games end next week as a gesture of goodwill to host China. But officials say that it's possible an invasion could occur in the days ahead.

The Pentagon has ordered an additional 3,000 U.S. troops to Poland to reassure allies.