An intruder was able to scale a fence and enter the White House due to "critical and major failures" of the US Secret Service, an internal review has found.

Iraq war veteran Omar Gonzalez was accused of breaking into the heavily-guarded complex on September 19 armed with a knife in one of the most significant security breaches since President Barack Obama took office in 2009.

The suspect was not stopped until he entered the main floor of the White House.

In addition to the knife he was carrying, officers found more weapons in his car.

US Secret Servie director Julia Pierson resigned following a series of breaches. Credit: Reuters

The incident helped spur the resignation of Secret Service director Julia Pierson.

US Department of Homeland Security suggests the suspect climbed over the seven-foot (two-metre) fence where an ornamental spike was missing.

Several uniformed Secret Service agents were stationed in the area but were unable to see the intruder because of a construction project along the fence line, the report stated.

An officer stationed on the White House driveway with a guard dog was making a call on his personal mobile at the time of the incident and was not wearing his earpiece.

Oscar Gonzalez was accused of breaking into the White House armed with a knife. Credit: REUTERS/New River Regional Jail/Handout

After spotting the intruder, the officer was aid to have moved towards Gonzalez, giving the dog the command to apprehend the suspect.

But the canine "did not have enough time to lock onto" him and "may not have seen him at all," according to the report's executive summary.

It concludes:

This report indicates that the Secret Service's response at the White House was significantly hampered on September 19 because of critical and major failures in communications, confusion about operational protocols and gaps in staffing and training.

US Department of Homeland Security report