President Barack Obama on Tuesday called out prominent Republicans for continuing to endorse Donald Trump in his bid for the White House.

"I think the Republican nominee is unfit to serve as president. I said so last week and he keeps on proving it," Obama said on Tuesday during a joint press conference with Singaporean Prime Minister Lee Hsien Loong at the White House.

Obama said Trump is "woefully underprepared" to be president. "He doesn't appear to have basic knowledge on critical issues in Europe, in the Middle East, in Asia," Obama said.

Obama said this opinion of Trump is shared by prominent Republicans.

"I think what's been interesting is the repeated denunciations of his statements by leading Republicans including the speaker of the House, the Senate majority leader and prominent Republicans like John McCain," he said. "The question I think that they have to ask themselves is, if you are repeatedly having to say, in very strong terms, that what he has said is unacceptable, why are you still endorsing him?"