WASHINGTON - US President Barack Obama on Tuesday called Donald Trump "unfit" to be US president amid bipartisan outcry following the Republican presidential nominee's feud with the family of a fallen US Muslim soldier.

"I think the Republican nominee is unfit to serve as president," said Obama here at a press conference. "I said so last week and he keeps on proving it."

"The notion that he would attack a Gold Star family that has made such extraordinary sacrifices on behalf of our country, the fact that he doesn't appear to have basic knowledge around critical issues in Europe, in the Middle East, in Asia, means that he's woefully unprepared to do this job," said Obama.

Trump provoked bipartisan blame over the weekend after he derisively answered criticism from Khizr Khan, the father of a Muslim American solider killed by a suicide bomber in Iraq.

Khan, during last week's Democratic National Convention, blasted Trump for his divisive remarks and proposal to temporary ban Muslims entering the country and divisive tone.

The bellicose Republican standard-bearer responded by implying that Ms. Khan, who accompanied his husband on stage on the final day of the Democratic National Convention, was forbidden to speak.