West Ham say they are aware of a video that appears to show Declan Rice accusing the referee of "corruption" after their Europa League semi-final defeat by Eintracht Frankfurt.
The video appeared to be taken in the tunnel following Thursday's game.
Rice is filmed shouting expletives at referee Jesus Gil Manzano.
Hammers boss David Moyes and defender Aaron Cresswell were sent off in the 1-0 defeat that sealed a 3-1 loss on aggregate.
"The club is aware of the video circulating on social media but will not be commenting further until the referee's report has been considered," the club said.
"Ref, ref, it's so poor, all night, it's so bad," England midfielder Rice says in the video which has been widely circulated online.
"How can you be that bad, honestly? You've probably been paid."
During a post-match interview with BT Sport, Rice branded some of the decisions made on Thursday night "poor".
Moyes, speaking on Friday afternoon, said Rice's reaction showed how much the players cared about the match and the loss.
"We felt as if we had been treated quite badly overall and that is what happens," he said.
"If you know about football, everybody is emotional, so unless you're in the heat of it sometimes people don't understand how it feels.
"Last night, we had a group of players who were really connected, trying to win the game and felt it was very difficult under lots of different circumstances."
West Ham believe Rice's comments were through frustration at the referee's overall performance rather than one specific incident.
Under Uefa's guidelines a player can be banned for three matches if he is found to have insulted a match official.
However, any punishment is dependent on what the referee's report says.
Moyes said there is "not a chance" that Rice will be punished by the club, adding: "If Uefa punish him that's down to them, but not on my watch."
Moyes apologised after the match after being sent off for kicking a ball at a ball boy.
Moyes will have to serve a one-match ban because of the red card but there is a possibility that could be extended if Uefa feel it is warranted.
It is also likely Eintracht will be charged by Uefa for the pitch invasion that followed the final whistle.
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