The Islamic State (ISIS) has issued a threat to all four UEFA Champions League (UCL) quarter-final matches this week, increasing security fears throughout Europe.

The warning, issued by ISIS mouthpiece Al-Azaim Foundation, included a banner with the slogan “Kill them all” beside to the names of the four stadiums hosting the matches.

This comes just three weeks after a deadly ISIS attack on Moscow’s Crocus City Concert Hall, which killed 143 lives.

However, authorities in the affected cities have responded swiftly. Spanish police have significantly bolstered security measures for tonight’s Real Madrid vs. Manchester City match and Wednesday’s Atletico Madrid vs. Borussia Dortmund game. Over 2,000 National Police and Civil Guard agents will be deployed for surveillance in Madrid alone.

UEFA, the governing body of European football, has acknowledged the threats and confirmed all matches will proceed as planned with “appropriate security arrangements in place.” The extent of additional security measures around London’s Emirates Stadium, where Arsenal takes on Bayern Munich tonight, remains unclear.

In a statement, UEFA said: Uefa is aware of alleged terrorist threats made towards this week’s Uefa Champions League matches and is closely liaising with the authorities at the respective venues.

“All matches are planned to go ahead as scheduled with appropriate security arrangements in place.”