The US Secret Service implemented tougher disciplinary measures after preliminary findings from an internal investigation found agents missed an intruder at national security adviser Jake Sullivan's home in part because they were using their personal phones, people briefed on the matter said.

Secret Service Director Kimberly Cheatle in recent days ordered increased penalties for employees who violate policies on duty, including the use of personal devices while on the job.

The moves are partly in response to initial findings of an internal investigation following the April incident at Sullivan's home, when agents on his protective detail failed to see an intruder enter and exit, the sources said.

A law enforcement official familiar with the internal investigation said the agents on duty that night and their supervisors, are likely to be subject to disciplinary action, including an evaluation of whether they can maintain their federal security clearance, a requirement for their positions.

The incident at Sullivan's home occurred in the early morning hours in late April. Sullivan confronted the intruder inside the home and later told investigators that he believed the person was intoxicated and entered the home by mistake. Sullivan and his family were unharmed.

The internal investigation found the agents were distracted and on their personal phones while on duty and never saw the unidentified intruder, who was later seen on surveillance video entering and exiting the property, a person briefed on the matter said.

Cheatle this week ordered that disciplinary penalties be increased to up to 21-day suspensions, and up to removal for infractions that lead to operational failure. Those include for the use of personal phones or the use of alcohol while on assignments.

"The Director of the Secret Service Kimberly Cheatle issued a clear directive, emphasizing the importance of conduct and behavior in upholding our mission's excellence," said agency spokesman Anthony Guglielmi.

"We have zero tolerance for anything that jeopardizes operational success," he continued. "While human errors may occur, what sets us apart is our unwavering commitment to maintaining very high professional standards and ethics. This includes enhanced penalties for incidents involving alcohol and a strict policy regarding personal cell phone use while on duty."