Eight senior security officials told Reuters they were concerned that Trump's "shoot from the hip" style could pose national security risks, as they prepare to give him a routine pre-election briefing for presidential nominees.
They also cited his lack of foreign policy experience, and his little known team of foreign policy advisers.
"People are very nervous," one senior U.S. security official said.
However, the officials, who requested anonymity to discuss a political domestic issue, said they would not deviate from the usual "Top Secret" briefing format, to avoid any appearance of bias.
Current and formal officials also expressed concern over briefing Democratic presidential front-runner Hillary Clinton, according to Reuters.
They cited the scandal over her use of emails when she was secretary of State and her handling of sensitive information. She is currently facing an FBI probe over whether she compromised security and broke laws over her use of a private email server for government work at State.
"The only candidate who has proven incapable of handling sensitive information is Hillary Clinton," Michael Short, a spokesman for the Republican National Committee, told Reuters. "If there is anyone they should be worried about it is Hillary Clinton."
Some officials told Reuters that little harm could be done, because the traditional briefing is mostly a broad overview of national security issues and does not contain the most sensitive government secrets.
"If he reads the papers every day, he won't hear much that will surprise him," one U.S. intelligence official told Reuters.
Still, nominees are warned not to share the briefing's contents, which include detailed intelligence assessments.
The nominees are usually briefed shortly after their party nominating conventions in the summer, and are allowed to include one or two aides who must undergo security checks.
Rep. Adam Schiff (D-Calif.), the top Democrat on the House Intelligence Committee, told Reuters that he shared some of the concerns about Trump.
"I would be very concerned with Mr. Trump's ability to know what he can and can't discuss" publicly about the contents of an intelligence briefing, said Schiff, who supports Clinton for president.