London — Anonymous online supporters of the Islamic terror group ISIS have issued a threat to soccer stadiums across Europe ahead of major games in the quarterfinals of the UEFA Champions League — European soccer's biggest club competition.

A post disseminated this week by the pro-ISIS online media outlet Al Azaim Foundation showed graphic imagery of a gunman in a balaclava, with the message, "Kill them all," in large text. The post lists London's Emirates Stadium, Paris's Parc de Prince (sic), and Madrid's Santiago Bernabéu as targets.

All three stadiums are set to host major Champions League games, with tens of thousands of fans in attendance.

There were no related threats conveyed via any of the official social media accounts run by or known to be linked to ISIS.

UEFA, the body that runs the Champions League competition, said in a statement sent to CBS News on Tuesday that it was "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," the statement said.

Richard Barnes, a counterterrorism adviser who leads stadium security for London's Metropolitan Police, confirmed that the force was looking into the online threats ahead of the Champions League game between Arsenal and Bayern Munich at Emirates Stadium on Tuesday evening.

He told CBS News, however, that the online threats were "not a new tactic used by various terrorist groups to cause or raise alarm."

Barnes said the London police counterterrorism unit was "investigating this and they will also be engaging with internal and external partners and stakeholders to ensure this evening's fixture at Emirates Stadium is not affected."

French Interior Minister Gerald Darmanin said security would be "considerably reinforced" around Wednesday's Champions League game between Paris Saint-Germain and Barcelona, in the French capital, in response to the threat, according to the AFP news agency.

The threats and heightened security measures come just weeks after the bloody attack on the Russian capital's Crocus City Hall, which saw gunmen storm the concert venue before setting it on fire.

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A previously unheard-of ISIS Russia branch claimed responsibility for the attack, which left almost 200 people dead.

It also comes just days after an 18-year-old man from Idaho was arrested and accused of plotting to kill churchgoers in his town in the name of ISIS, according to court documents unsealed earlier this week.

ISIS has a history of bloody attacks on European soil, including the devastating, well-orchestrated assault on multiple locations around Paris in 2015. France's national soccer stadium, just north of Paris, was the only location outside the capital city that was attacked by the ISIS militants during that siege. It is not the same venue that was mentioned in the post on the pro-ISIS website this week.

CBS News' Khaled Wassef contributed to this report.

In:
• ISIS
• Terrorism
• Football
• ISIS-K
• UEFA Champions League
• European Union
• Soccer