"Mr. Trump rarely surprises me these days," Obama said in a press conference with Italian Prime Minister Matteo Renzi at the White House on Tuesday.
"I'm much more surprised and troubled that you have Republicans ... now supporting and in some cases echoing his positions."
He also rejected Donald Trump's allegations that the U.S election is being rigged to favour Democratic party candidate Hillary Clinton.
"I have never seen in my lifetime, or in modern political history, any presidential candidate trying to discredit elections and the elections process before even elections have taken place. It's unprecedented," Obama said.
"I'd invite Mr. Trump to stop whining and try to make his case to get votes."
With three weeks to go until the U.S Presidential elections, the two leaders (Obama and Renzi) also made remarks that can only be interpreted as thinly veiled commentary on the candidacy of Donald Trump.
"America was built by immigrants. America is stronger because of immigrants. America is great because of immigrants," Obama said.
Renzi spoke of building "bridges, not walls," and appealed for world leaders to look to the future, and not just the past.
"There are a lot of people who think politics is only about screaming, fighting each other, creating division. You’re different, Mr. President," Renzi said. "We're different."