"If these claims are confirmed, it would mean that the situation is extremely serious," Schulz said at the opening of a session of the European Parliament in Strasbourg.
It would be "a terrible blow for relations between the European Union and the United States," said the social-democrat, who had already voiced heavy criticism.
"I understand full well that measures have to be taken to fight against terrorism and that intelligence activities are necessary but I do not think we are preparing terrorist attacks in the European institutions," he said.
Schulz said he had met with the US ambassador to the European Union and had asked him for answers "as soon as possible".