Thousands of U.S. nationals have signed up to join the Ukrainian military as the country attempts to fight off the Russian incursion, reports say.

Ukraine has received more than 3,000 applications from people in the United States who want to fight, several of whom are military veterans, a defense official with the Ukrainian Embassy in Washington, D.C., told the Military Times on Thursday.

“If you are determined to go, be professional, responsible, and realistic in your expectations,” a former SEAL-turned-CIA officer who goes by Frumentarius said. “Follow the Ukrainian government’s advice about how to officially register, and do not go there telling them where and how you want to participate in the fighting. … You are not Rambo, there to single-handedly slay Russians and post your selfies.”

Those U.S. fighters are a part of the roughly 16,000 foreigners who have volunteered to join the military, Ukrainian President Volodymyr Zelensky announced in a video posted to his Telegram channel Thursday.

“We have nothing to lose but our own freedom,” the president said.

Other members of his Cabinet have reiterated the calls for foreigners to come and join the fight, though the U.S. government has advised against it.

“This is not the place for Americans to be in Ukraine right now, and the State Department has made that very clear in urging over so many weeks for Americans that are in Ukraine to leave, and urging Americans not in Ukraine not to go,” Pentagon spokesman John Kirby told reporters Friday.

“Should Americans want to help Ukraine, and it’s laudable that they do, but the best way to do that is to find ways to contribute to the many nongovernmental and humanitarian organizations that are trying to alleviate what has now become a very acute humanitarian crisis in Ukraine,” he added.

Additionally, the State Department warned that getting “recruited or hired” to join a foreign military while in the U.S. would violate a Supreme Court decision.

Other countries have also faced the question of whether they should allow, encourage, or prevent their civilians from joining the Ukrainian forces. Latvia’s parliament voted unanimously to allow its citizens to do it, while Denmark and Canada decided to warn their people of the risks but not stop them from doing so, according to Reuters.

Ukraine’s General Staff of the Armed Forces detailed the process of how to apply in a Facebook post on Thursday instructing those interested to contact the Ukrainian embassies in their home countries and apply in written form to join the International Legion of Territorial Defense Forces of Ukraine under a voluntary contract.

Since Russia took military action more than a week ago, the invading forces have had more success in the southern parts of the country, where there is more infrastructure set up to resupply troops and shorter routes to do so. Forces in the north, which are seeking to capture the capital, Kyiv, have been met with more resistance than expected and have been “stalled” roughly 15 miles outside the city for multiple days, a senior defense official told reporters Friday.

Various countries have provided military support to Ukraine, though they have refrained from sending troops to supplement the Ukrainian forces.