The man the Trump administration has put forward to coordinate billions of dollars in assistance to migrants is alleged to have said on Twitter that Islam is a violent religion and that Christiansshould be given preferential treatment.
The Washington Post says it has reviewed tweets, social media posts and radio appearances by Ken Isaacs, a vice president of the Christian relief organisation Samaritan's Purse, and found a number of derogatory comments about Muslims.
The US media organisation claims that in June last year, following the terrorist attack in London Bridge, Mr Isaacs commented on a CNN International story claiming that the attack was "exactly what the Muslim faith instructs the faithful to do".
In another tweet he is alleged to have said: "If Islam is a religion of peace, let's see 2 million Muslims in National Mall marching against jihad & stand for America! I haven't seen it!"
And in response to Barack Obama's position on Syrian refugee relief in 2015, Mr Isaacs tweeted: "Refugees are 2grps. Some may go back and some can't return. Christians can never return. They must be 1st priority."
Mr Isaacs has since made his Twitter account private, but screenshots of his tweets are being shared on the social networking site.
He was announced on Thursday as the Trump administration's nominee to become director general of the United Nations' International Organisations for Migration (IOM), which has an annual operating budget of almost $1bn.
In a statement to The Washington Post, Mr Isaacs said: "I deeply regret that my comments on social media have caused hurt and have undermined my professional record.
"It was careless and it has caused concern among those who have expressed faith in my ability to affectively lead IOM.
"I pledge to hold myself to the highest standards of humanity, human dignity and equality if chosen to lead IOM."
Mr Isaacs comments are putting him at risk of being the first US nominee since the late 60s to lose an election to become the head of the IOM by the group's voting members.
Eric Schwartz, president of Refugees International, told The Washington Post: "I don't know the nominee, but I've seen some of his statements and they reflect a troubling prejudice that is really incompatible with a position of leadership for the world's most important international migration agency.
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