The incident was confirmed airport officials.
"Don't know what happened or why they were sent back to Islamabad but there was no verbal spat or arguments," one official said on condition of anonymity.
A US embassy spokesman confirmed the incident and told Dawn.Com that four embassy officials had reached Karachi from Islamabad this morning to meet with officials from the US consulate in Karachi.
He said airport officials did not allow them to enter the city and they subsequently returned to Islamabad.