According to security officials speaking to the Associated Press, the Daesh terrorist group has a number of semiautonomous cells spread across Europe.
The warning comes from a number of European and Iraqi intelligence officials, as well as a French lawmaker familiar with jihadi networks.
Approximating at least 400 individuals, these fighters have been given orders to find the right time, place, and method to carry out their mission, according to officials.
Earlier on Wednesday, the US State Department issued a travel alert for all of Europe, warning that "terrorist groups continue to plan near-term attacks throughout Europe."
"European governments continue to guard against terrorist attacks and conduct raids to disrupt plots," the alert added.
Suicide bombers in Brussels attacked the Belgian capital's airport and a metro station on Tuesday, killing at least 30 people people and injuring approximately 260. Daesh claimed responsibility for the attacks.