Anderlecht have apologised for the behaviour of their fans during their 2-1 defeat by West Ham in the Europa Conference League on Thursday night.
Several flares were set off in the area where the Anderlecht fans were and one was thrown when the visitors pulled a goal back a minute from time.
Manager Felice Mazzu said he did not see the latter incident but a club spokesman condemned the behaviour.
"We want to apologise to West Ham," he said. "We don't tolerate this and strongly condemn this behaviour. We will take it up with Uefa and our own fans.