President Barack Obama said on Tuesday Donald Trump's attacks on the family of a Muslim U.S. Army captain killed in Iraq and his lack of understanding of critical issues means the Republican candidate is "woefully unprepared" to serve as president.

"There has to come a point at which you say 'enough,'" Obama said, addressing Republican leaders who he said have repeatedly denounced statements made by Trump but who continue to endorse him for president. Obama, a Democrat, was speaking at a White House news conference with Singapore's prime minister, Lee Hsien Loong.