WASHINGTON -- US President Donald Trump on Tuesday warned Russian President Vladimir Putin that he is "playing with fire" as the Russia-Ukraine conflict shows no immediate signs of ending.

"What Vladimir Putin doesn't realize is that if it weren't for me, lots of really bad things would have already happened to Russia, and I mean REALLY BAD. He's playing with fire!" Trump wrote in a Truth Social post Tuesday morning.

Trump on Sunday made harsh remarks on both Putin and Ukrainian President Volodymyr Zelensky.

"I'm not happy with what Putin's doing," Trump told reporters before he took Air Force One from Morristown, New Jersey back to Washington, DC.

"I don't know what the hell happened to Putin," he said.

Later on Sunday evening, Trump wrote in a Truth Social post that Putin "has gone absolutely CRAZY!"

He then turned to Zelensky, saying he was doing his country "no favors by talking the way he does," a reference to statements Zelensky made earlier on Sunday criticizing US silence on the latest Russian attacks.

The Trump administration has warned in recent months that it could walk away from the frustrating negotiations for a cease-fire between Russia and Ukraine.

In response to Trump's escalating criticism, Russian Security Council Deputy Chairman Dmitry Medvedev wrote in a social media post on Tuesday that "Regarding Trump's words about Putin 'playing with fire' and 'really bad things' happening to Russia. I only know of one REALLY BAD thing - WWIII. I hope Trump understands this."