MITCHELL FIELD, L. I., Sept. I1.--A mysterious round object "marching" along at 900 miles an hour five miles over the New Jersey coast was sighted by two Air Force jet pilots yesterday on their way from Dover, Del., to Mitchel Air Force Base. 

Although an Air Force spokesman described their report as "the usual flying saucer story," Lt. Wilbert S. Rogers of Columbia, Pa., who was piloting the T-33 jet training plane, and his passenger, Capt. Edward Ballard, Jr., emphasized that they were in no way sure that they did see a saucer in flight.  They were sure only that they had never seen anything like it before. 

Lt. Rogers reported that he was flying at 20,000 feet and 450 miles and hour when he and Capt. Ballard sighted the object about 8,000 feet below them.  It was descending in an arc or boomerang fashion dispelling the idea that it might be a weather balloon, one of the explanations Air Force and Navy officials have given for flying saucers. 

Lt. Rogers said Capt. Ballard suggested that they follow it, but although they followed it for thirty miles and got within 8,000 feet of it they could not keep pace as it moved from Sandy Hook to Red Bank, then headed out to sea at Asbury Park.  It seemed to have a constant speed and had dropped to an altitude of about 2,000 feet before it disappeared.  The pilot had kept it in sight for about two minutes. 

<>Air Force officials have refused to comment on the story but said that the two pilot would make an official report today on their return to Dover Air Base, where they are attached to the 148th Fighter Interceptor Wing.