The US Secret Service is investigating how an intruder entered US national security adviser Jake Sullivan's home last month without being detected by Secret Service agents guarding his home.

Sullivan, who has 24/7 Secret Service protection, was unharmed in the incident, according to Secret Service spokesman Anthony Guglielmi, who said the agency takes the matter seriously.

"Secret Service is examining a security incident that took place at a protectee site. While the protectee was unharmed, we are taking this matter seriously and have opened a comprehensive mission assurance investigation to review all facets of what occurred," Guglielmi said in a statement.

"Any deviation from our protective protocols is unacceptable and if discovered, personnel will be held accountable. Modifications to the protective posture have also been made to ensure additional security layers are in place as we conduct this comprehensive review," Guglielmi said.

The Washington Post first reported the incident.