Chertoff's comment to the newspaper's editorial board came as ABC News also reported the White House has called a security meeting Thursday to assess the risk to government facilities.

"I believe we are entering a period this summer of increased risk," Chertoff said. "Summertime seems to be appealing to them (al-Qaida)."

He said there is an intelligence assessment "not of a specific threat, but of increased vulnerability," the newspaper said.

He said information Britain shared with the United States over recent foiled terror attacks showed changes in the terrorist' tactics.

"If you look at that picture you see an enemy that is improving itself just as we're improving ourselves," he said. "They can't afford to remain static just as we can't afford to remain static."

Administration spokesman Tony Snow was asked at the daily White House briefing Thursday about Chertoff's use of the term "gut feeling" to describe his sense of an increased risk.

"I'm glad we got a Homeland Security secretary who worries about it all the time," said Snow.

When a reporter asked whether President George W. Bush shared Chertoff's gut feeling, Snow joked: "I don't want to get into gastrointestinal descriptions."