A member of a Yale University fraternity turned dark-skinned students away from a party this weekend, telling them: "No, we're only looking for white girls."

That's according to a student who said she was standing next to him on the stairs when he held his hand out to block a group from entering the Sigma Alpha Epsilon party.

Those girls looked startled and walked away, said Sofia Petros-Gouin, a freshman at Columbia who was visiting friends at Yale and described a stairway crowded with people trying to get into the Halloween party, and the fraternity member, who was white, repeatedly saying, "White girls only," and letting blonde women go in.

"I was shocked," she said. "I was disgusted."

The allegation, which was echoed on social media in several complaints about similar incidents at Yale SAE, was especially charged because Sigma Alpha Epsilon has been repeatedly accused of having racist traditions as part of the fraternity culture. The national chapter announced initiatives in the spring designed to ensure that racist behaviour is not tolerated in its chapters.

It opened up a heated conversation at Yale, with some students saying it shows blatant racism on campus, and others questioning whether it could have really happened.

The president of Yale's SAE house, Grant Mueller, denied to the Yale Daily News that members racially discriminated against students at the off-campus event.

A member who was at the party and requested anonymity because chapter rules discourage speaking to the media gave an entirely different account than Petros-Gouin.

He said they always ask for Yale IDs and let everyone in before a party gets crowded, and after that a line forms. That night, no one with a Yale ID was turned away before 11.15pm, he said. Yale and New Haven police had responded to noise complaints at the party, and frat members were told not to let anyone else in at that time to avoid crowding.

He said numerous students have said a woman who was denied entrance angrily challenged the student who stopped her, screaming: "It's because I'm black, isn't it?"

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It was incredibly uncomfortable, he said. That brother is African-American, he said, and others working the door at the time are Portuguese and Costa Rican. He described the chapter as racially diverse.

Some of those who are black have been called "Uncle Tom" today, he said, by members of Yale's black community, and feel like they are being forced to choose between siding with the fraternity or others of their race.

He and another student who attended the party said the crowd inside was diverse, representative of the student body at Yale.

Brandon Weghorst, a spokesman for SAE's national office, also said the frat members had stopped admitting guests after being ordered so by police enforcing a noise complaint.

Petros-Gouin said the frat member in question is white. She said several of her friends from Columbia were visiting friends at Yale this weekend, and about 10.30pm or 11pm tried to get into the SAE party, which had several levels of screening.

Students without a Halloween costume or a Yale ID were turned away but her group was told that they could go in if they had a Columbia ID, she said.

She wasn't surprised that the fraternity would be screening, or that pretty girls might have a better chance of getting in.

But she was surprised when "a group of girls came up who were predominantly black and Hispanic," she said.

"He held his hand up to their faces and said, 'No, we're only looking for white girls'… He pulled a blonde girl up from the bottom of the stairs - over some people - pushed her inside and said: 'We are looking for white girls only, white girls only.' No brothers corrected him."

He repeated it, she said, and a group of women on the stairs raised their hands and kind of jumped up, wanting to get in.

"He pulled that group up who volunteered because they were white, and said, 'Yeah, that's what we're looking for'," she said.