President Donald Trump was impressed by Ukraine's surprise drone attack against Russia, according to sources, but he's concerned it will complicate efforts to reach a ceasefire.

The meticulously planned remote operation against Russian air bases showed Ukraine may have more "cards" than Trump previously believed, and sources who discussed the attack with him told Axios he described their move as "strong" and "badass" but basically dared Vladimir Putin to escalate in response.

"'Pretty strong,' Trump told one confidant, who relayed his remarks to Axios. "'He thought it was badass,' a second source said of Trump's reaction."

Trump spoke to the Russian president on Wednesday and said Putin had told him "very strongly" that he would forcefully respond to the drone attack. "It was a good conversation, but not a conversation that will lead to immediate peace," Trump said of their call.

The attack concerned the White House because Ukraine had targeted long-range bombers that can carry nuclear weapons, and one U.S. official described the situation as "a very dangerous point" in the ongoing conflict.

"The drone attack didn't surprise him, because this is what happens when the war continues, but he is concerned that it will prolong the fighting," said a senior White House official. "He wants the fighting to stop, so it disappoints him whenever there is an attack like that."

Despite those concerns, Trump told his advisers the operation was impressive.

"From an international perspective, you've got a chihuahua inflicting some real damage on a much bigger dog," said one Trump adviser.