United Soccer League championship authorities have launched an investigation after the San Diego Loyal walked off the field in protest during a game on Wednesday alleging that a player had been subjected to homophobic abuse.

San Diego, coached by former US international star Landon Donovan, abandoned the game against Phoenix Rising in California after claiming that openly gay midfielder Collin Martin was targeted with an anti-gay slur.

The USL Championship, a professional league ranked below Major League Soccer, said in a statement it was investigating the incident.

"We are aware of the alleged use of a homophobic slur in tonight's match between San Diego Loyal SC and Phoenix Rising FC," the league said.

"Foul and abusive language of any type has absolutely no place in our society and will not be tolerated in USL matches. An investigation is currently underway to determine the facts surrounding the incident and more information will be provided as soon as it is available."

The incident comes just a week after San Diego forfeited a game after one of their players was racially abused by a Los Angeles Galaxy II opponent.

"Last week we made it loud and clear that we do not stand for racism or homophobia," San Diego Loyal wrote. "Nothing has changed this week."

Wednesday's flashpoint occurred after Martin was sent off with San Diego leading 3-1. Several players could be seen approaching the referee before walking off.

In a video statement posted, Donovan later stood behind his players for their decision to walk off, even though abandoning the game effectively ended the team's chances of reaching the post-season playoffs.

"Our guys, to their immense credit, just said, 'We're not going to stand for this,'" Donovan said. "They were very clear in that moment that they were giving up all hopes of making the playoffs, even though they were beating one of the best teams in the league handily.

"But they said that doesn't matter, there's things more important in life and we have to stick up for what we believe in. And so they made the decision to walk off."

Phoenix, meanwhile, said they were investigating the incident but said in a statement the player accused of uttering the homophobic abuse "vehemently denies these allegations."