Chelsea stars Diego Costa and Oscar had to be separated by team-mates as a training-ground clash threatened to turn into an ugly brawl, according to reports on Thursday.

Spain striker Costa reportedly reacted angrily to an aggressive knee-high challenge from Brazil midfielder Oscar, who was said to be furious with a previous tough tackle from the notoriously temperamental Spain striker.

The Daily Telegraph claimed "the pair squared up and had to be pulled apart by their shocked team-mates before angry words could turn into a physical scrap".

The ugly scene is the latest bad news for the troubled Premier League champions in a turbulent season which saw boss Jose Mourinho sacked as manager last month following the disintegration of his relationship with several members of the squad.

However, Chelsea interim manager Guus Hiddink is believed to be relatively unconcerned by the incident as he prepares for Sunday's FA Cup third-round clash against third-tier Scunthorpe United at Stamford Bridge.

Oscar also tried to play down the controversy on his Twitter account.

"For those who are talking to me and @diegocosta struggle today in training he's lying, he's one of my best friends and never fight," he tweeted.

Costa and Oscar's clash comes after Chelsea youngster Charly Musonda was involved in a similar incident with Cesc Fabregas just prior to Mourinho's dismissal.