The US secretary of state had been planning to leave Brussels late today on his US Air Force Boeing C-32AKA, a military variant of the 757, after attending a series of NATO meetings.
But instead he and his entourage were heading to the main rail station to catch a train to Paris.
"Due to a potential delay in flights departing Brussels as a result of an air traffic control strike, Secretary Kerry will take the train from Brussels to Paris this evening," his spokeswoman Jen Psaki said.
He ended a press conference at the NATO headquarters saying he "had to go" as Psaki added: "Sorry we have to catch a train."
It was the latest stop on a whirlwind tour which has taken him so far to Cairo, Baghdad, Arbil and Brussels.