The US helicopters entered into the Pakistani airspace at around 10.00 pm and hovered over the area for about 15 minutes, the Daily Times quoted the sources as saying. However, they added, the choppers returned to Afghanistan without attacking any target. This was not the first occasion when US drones or helicopters had intruded into Pakistani air space. A few such incidents had come to light in the recent months, with Pakistani authorities taking objection to it and warning the US against it. On a few occasions, the US aircraft had even attacked targets on Pakistani soil, killing its Frontier Corps (FC) troops.