US President Barack Obama and Singapore Prime Minister Lee Hsien Loong (L) hold a joint press conference in the East Room of the White House, August 2, 2016 in Washington, DC. (AFP photo)

US President Barack Obama has issued a sharp criticism of Republican presidential nominee Donald Trump, calling him “unfit” to be president.
Obama made the comments on Tuesday during a White House news conference with Singapore's prime minister, Lee Hsien Loong, after Trump criticized an American Muslim family whose son was killed in Iraq.
Obama also challenged Republican leaders to withdraw support for Trump for his criticism of Khizr and Ghazala Khan, who appeared at the Democratic National Convention (DNC) last week.
During the DNC address, Khizr Khan denounced Trump as unpatriotic and selfish over his divisive rhetoric against immigrants and Muslims.
Trump responded, in part, by suggesting that Ghazala Khan, the mother of the fallen soldier, was silent during her husband’s speech at the DNC because she was not “allowed” to speak.
Ghazala Khan said that she did not intend to speak during the convention because she would get emotional.

This file photo taken on July 28, 2016 shows Khizr Khan holding his personal copy of the US Constitution while addressing delegates on the fourth and final day of the Democratic National Convention at Wells Fargo Center in Philadelphia, Pennsylvania. (AFP photo)

"The notion that he would attack a Gold Star family that had made such extraordinary sacrifices on behalf or 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," Obama said of Trump.
Families who lose sons or daughters who serve in the US military are called Gold Star families.
Obama, who is a Democrat, noted that Republican leaders, including Senate Republican leader Mitch McConnell, Speaker of the House of Representatives Paul Ryan, and senator John McCain, a one-time Republican presidential nominee, have criticized Trump for his remarks but have stood by their endorsements of him.
"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?" Obama said.
"What does this say about your party that this is your standard-bearer?" he asked.
Obama has endorsed the Democratic nominee and his former secretary of state, Hillary Clinton.
A Gallup poll released early last month found that Trump and Clinton are among the worst-rated presidential candidates of the last 70 years.

In a searing denouncement, President Barack Obama castigated Donald Trump as "unfit" and "woefully unprepared" to serve in the White House. He challenged Republicans to withdraw their support for their party's nominee, declaring "There has to come a point at which you say enough."
While Obama has long been critical of Trump, his blistering condemnation today was a notable escalation of his involvement in the presidential race. Obama questioned whether Trump would "observe basic decency" as president, argued he lacks elementary knowledge about domestic and international affairs and condemned his disparagement of an American Muslim couple whose son was killed while serving the US Army in Iraq.
A chorus of Republicans has disavowed Trump's criticism of Khizr and Ghazala Khan and the Republican nominee's calls to temporarily ban Muslims from coming to the US. But Obama argued that isn't enough.

Trump's response? On Twitter, he said, "President Obama will go down as perhaps one of the worst president in the history of the United States!""If you are repeatedly having to say, in very strong terms, that what he has said is unacceptable, why are you still endorsing him?" Obama asked during a White House news conference. "What does this say about your party that this is your standard-bearer?" No prominent Republican lawmaker responded to Obama's challenge.
Obama's harsh comments were an attempt to raise the stakes for Republicans, suggesting their support for Trump will taint their party for years to come. His statements also dovetail with Democratic nominee Hillary Clinton's efforts to reach out to Republican voters - particularly women - who may be so upset by Trump that they're willing to look past policy differences and questions about Clinton's character.
Republican presidential candidate Donald Trump, picture with his son Barron, has been slammed as "unfit" and "woefully unprepared" to serve in the White House. Photo / AP
The president - who is enjoying heightened popularity in his eighth and final year in office - plans to campaign robustly for Clinton through Election Day. He and first lady Michelle Obama spoke at last week's Democratic convention in Philadelphia.
The Khans also appeared at the convention, with Khizr Khan telling the story of his son, US Army Captain Humayun Khan, who was posthumously awarded a Bronze Star and Purple Heart after his death in 2004. Khan criticised Trump's position on Muslims and asked whether the real estate mogul had read the Constitution.
For most politicians, tangling with a bereaved military family would be out of bounds. But Trump dove in, questioning why Ghazala Khan did not speak, implying her religion prevented her from doing so, and saying he was "viciously attacked" by Khizr Khan.
Trump's criticism was part of a familiar pattern: He can't let go of a perceived slight, no matter the potential damage to his presidential campaign or political reputation.
Those who have worked with him say that in private meetings he can often appear amenable to putting a controversy aside. But the businessman can quickly be drawn back in by an interview, especially if he believes he's already answered a question, or if he grows irritated by commentary on cable television.
Trump's unwillingness to let the matter subside sparked outrage Monday from several Republicans.
John McCain, a former prisoner of war, said Trump did not have "unfettered license to defame those who are the best among us."
Mike Coffman, a vulnerable Republican in a competitive Colorado district, said he was "deeply offended when Donald Trump fails to honour the sacrifices of all of our brave soldiers who were lost in that war." Senator Roy Blunt said the Khans "deserve to be heard and respected."
Obama argued those denunciations "ring hollow" as long as Republicans continue to back Trump in the White House race.
Senator Mark Kirk, who is facing a tough re-election fight in Illinois, rescinded his endorsement of Trump in June after the GOP nominee criticised an American-born judge's Mexican heritage. Others, including Senate Majority Leader Mitch McConnell and House Speaker Paul Ryan, have broken with the nominee on individual issues but continue to back his candidacy.
Ahead of Obama's remarks today, Republican Richard Hanna of New York said he would be voting for Clinton. Hanna, who is retiring from Congress, had previously said he couldn't back Trump.