By Gram Slattery

PALM BEACH, Florida, Jan 3 (Reuters) - U.S. President Donald Trump said on Saturday he was frustrated with Russian President Vladimir Putin over the continued war in Ukraine, repeating that he had initially believed that ending the war would be easy.

"I'm not thrilled with Putin. He's killing too many people," Trump told reporters at his Mar-a-Lago resort in Palm Beach, Florida, during a press conference to discuss the overnight U.S. military operation in Venezuela and capture of its President Nicolas Maduro.

Trump said he did not discuss Maduro with Putin when the two leaders spoke by telephone for over two hours on Monday, shortly before the U.S. president met with Ukrainian leader Volodymyr Zelenskiy in Palm Beach.

"We never spoke about Maduro," Trump said when asked if he and Putin had discussed the Venezuelan leader.

Russia's foreign ministry said on Saturday it was extremely concerned by reports that Maduro and his wife had been removed from the country during "aggressive actions" by the United States, and called for the pair's immediate release.

U.S. Secretary of State Marco Rubio told reporters that Maduro had rejected multiple chances to flee the country, and some reports suggested weeks ago that he could have headed to Russia or closely allied states.

Trump on Monday initially expressed sympathy for Russian allegations that Ukrainian drones attacked one of Putin's residences. He later appeared more skeptical and shared a New York Post editorial accusing Russia of blocking peace in Ukraine.

The Wall Street Journal later reported that U.S. national security officials had found Ukraine did not target Putin or one of his residences in a drone strike.

Trump has repeatedly expressed frustration about the lack of progress toward ending the war in Ukraine, which began with Russia's invasion in February 2022, focusing heavily on what he calls tens of thousands of troops being killed on both sides.

(Reporting by Andrea ShalalEditing by Rod Nickel)