Obama says Trump 'unfit' for presidency

President Barack Obama offered one of his sharpest denunciations of Donald Trump to date Tuesday, declaring the Republican nominee entirely unfit to serve as president and lambasting Republicans for sticking by their nominee.

The strong rebuke in the White House East Room came after Trump's criticism of the family of a slain Muslim US soldier, along with comments that displayed apparent confusion related to the Russian incursion into Ukraine.

"The Republican nominee is unfit to serve as president," Obama said at a White House news conference with the Prime Minister of Singapore. "He keeps on proving it."

The Trump campaign responded by going after the Democratic nominee as well as the President.

"Hillary Clinton has proven herself unfit to serve in any government office," a Trump statement said, listing a number of policy concerns. "Obama-Clinton have single-handedly destabilized the Middle East, handed Iraq, Libya and Syria to ISIS, and allowed our personnel to be slaughtered at Benghazi."

Later Trump in an interview with WJLA said of Obama: "He's a terrible president. He'll probably go down as the worst president in the history of our country. He's been a total disaster."