A US F-15 fighter jet has intercepted a single-engine aircraft near Chicago in the third such incident during a weekend visit by President Barack Obama and his family, authorities say.
The North American Air Defence Command (NORAD) says the fighter responded after a Cessna 172 violated a temporary flight restriction.
"After intercepting the single engine aircraft, the F-15 escorted it out of the flight restricted area without further incident," the Colorado-based command said.
The incident, which occurred at 4pm on Sunday (0700 Monday AEST), followed two separate interceptions on Saturday near Chicago.
Those aircraft also were escorted out of the restricted area without incident, NORAD said.
Obama was in Chicago to spend the weekend there with his family and to attend the wedding of the daughter of Valerie Jarrett, one of his senior advisers.