More than 3,000 Europeans have joined the Islamic fighters in Iraq and Syria, Gilles de Kerchove, EU's anti-terrorism chief, told the BBC, adding that the airstrikes could enhance the risk of retaliatory attacks in Europe. The U.S. has been conducting airstrikes in Iraq and Syria to fight the advance of the Islamic State group, formerly known as ISIS, while the UK's parliament is debating whether it should join the air campaign. The U.S. said that more than 40 countries have offered to join in the anti-ISIS coalition.

Kerchove said that the 3,000 fighters also included those who have returned from the region and the ones who have been killed. The U.S. forces have carried out nearly 200 airstrikes in Iraq since August and, since Monday, U.S. forces have also started striking ISIS strongholds in Syria. According to the estimates from the CIA, cited by the BBC, nearly 31,000 people may be fighting for the Islamic State group, while Kerchove added that the group’s move of declaring a caliphate in June may have helped the group to get more support from Europe.

"If you believe in this, probably you want to be part of it as early as possible," Kerchove said, according to BBC, adding that the airstrikes may prompt al Qaeda to take some action, to try getting ISIS out of the limelight.

The group has conducted three executions so far while a French man, who was kidnapped on Sunday in Algeria, was executed by Jund al-Khilafa, a militant group supporting ISIS, in a video released Wednesday. France, which had begun its airstrikes in Iraq last week, said Thursday that Paris has not completely ruled out joining in airstrikes in Syria, and that it is still evaluating whether it should expand its operations.

"Three days ago [ISIS] issued a statement saying there would be retaliation against the coalition. A French man was kidnapped in Algeria and he has been beheaded. So they did what they announced," Kerchove said, according to BBC.

Rear Admiral John Kirby, Pentagon spokesperson, said, according to BBC, "Even after the hits they've taken, and they have been hit, they still have financing at their fingertips," adding that it was too soon to be talking of winning against the group.