Terrorists could launch attacks on the U.S. mainland before the nation holds its presidential election this November, America's leading intelligence official has warned.

Speaking to Newsmax on Monday, Director of National Intelligence James Clapper said the attacks could be launched by terrorist groups like the Islamic State (ISIS) or al-Qaeda.

Clapper was responding to a question asked by Newsmax at a Washington, D.C., press breakfast hosted by the Christian Science Monitor.

"Whether or not they [terrorists] might do something that might have a bearing on the election, well, they could — particularly if they do something in this country," he replied.

"It would probably have some impact. It's kind of an imponderable but it could certainly influence how people vote in this election."

As to whether the terrorist groups might prefer Donald Trump to help rally jihadists, Clapper said: "I cannot point to any evidence that would indicate a preference on the part of ISIS as to who is elected in our presidential election."

Clapper said, however, he was concerned about some of the rhetoric in this year's presidential campaign.

"Certainly you worry about rhetoric on the campaign trail," he said. "I think the history has been that once a president is inaugurated and is in office and realises the burden and the responsibilities of the position, I think that has a tempering effect on anyone."

Speaking to reporters during the press breakfast, Clapper said the intelligence community's task of uncovering the plans of terrorist organisations like the ISIS has become even more difficult since these organisations have accelerated the sophistication of encryption technologies by "about seven years," the Christian Science Monitor reports.

New, commercially available encryption software "had and is having major, profound effects on our ability" to collect intelligence, "particularly against terrorists," he said.

Clapper revealed that the ISIS is now "the most sophisticated user by far of the Internet." ISIS privately purchases software to ensure "end-to-end encryption" of its communications, he said.

Clapper also warned on Monday that ISIS has clandestine cells that are plotting more terrorist attacks in Germany, Italy, and England.

He revealed that U.S. intelligence officials have learned new things about ISIS. For starters, they are "very op-sec conscious," Clapper said, using military parlance for "operational security."

It is also clear that the ISIS is taking advantage of the migrant crisis in Europe, he added.