Russian President Vladimir Putin has threatened to use nuclear weapons in Ukraine and cautioned against calling his bluff. The threat is even more plausible since Russia annexed four regions of Ukraine in September, a move the international community has condemned as illegal.

However, Putin has said he will do whatever it takes to defend Russian territory, the four annexed regions included.

“I want the Kyiv authorities and their real masters in the West to hear me so that they remember this. People living in Luhansk and Donetsk, Kherson and Zaporizhzhia are becoming our citizens. Forever,” Putin said in a speech.

Experts said if Russia follows through on the threats, the first trike would likely be a symbolic target.

Dr. Rod Thornton, a security expert at King’s College London, told Forbes Putin would likely avoid striking a Ukrainian city to minimize fallout. Instead, Thornton said a target like Snake Island could be attacked with nuclear weapons to show the leader of the Kremlin is serious.

Thornton added that Putin might be more likely to deploy tactical nuclear weapons, smaller devices designed for use on the battlefield. He noted, however, they could be just as devastating.

“On many fronts, Putin is under pressure,” Thornton told Forbes. “The more desperate Putin becomes, the more he’s pushed on the back foot, the more likely it becomes that a nuclear weapon is used.”

The Russian army has suffered unexpected losses in Ukraine and troops are said to be “exhausted,” according to The Washington Post.

The Post reported that a top spy chief from the United Kingdom is expected to give a speech confirming the state of Russian troops. The outlet said it had obtained a “preview” of the address scheduled to be delivered by Jeremy Fleming, who leads Britain’s Government Communications Headquarters (GCHQ).

“Russia’s forces are exhausted. The use of prisoners to reinforce, and now the mobilization of tens of thousands of inexperienced conscripts, speaks of a desperate situation,” Fleming is expected to say when addressing the Royal United Services Institute (RUSI).

Fleming is also expected to say Putin’s “decision-making has proved flawed.”