A man was charged last night with committing a sexual assault near the Aspen Hill area of Montgomery County where a series of 16 rapes has created intense concern among residents. The man was arrested after a high-speed chase that ended in a head-on collision with a police car.

County police said James Edward Flanagan, 29, of Silver Spring was charged with committing a first-degree sex offense. Such an offense involves force or the threat of force; police would not give any details of the alleged assault or its victim.

Police also emphasized that an intensive investigation of the series of rapes in the Aspen Hill area of the county is continuing. "Nobody wants to let their guard down," said police Lt. Joseph Hancock. "Citizens should continue to be alert to suspicious actions in the area where these assaults have taken place," said police spokesman Philip B. Caswell.

A series of 16 sexual assaults since March 1981 in the Aspen Hill area has been receiving the police department's top priority effort, Police Chief Bernard D. Crooke said at meeting Tuesday night. That meeting, at which Crooke and other police officials outlined efforts to catch what is believed to be a single assailant, attracted about 600 persons from Aspen Hill.

Police said they got a call last night reporting a sexual assault in the Bel Pre Road-Norbeck Road area in western Wheaton about 7:05 p.m. Investigators said the location is just north of the Aspen Hill area where the sexual assaults on women were reported.

Police, supplied with a description of a man driving a black Jeep, converged on the area and an officer driving a scout car spotted such a vehicle and stopped it at Georgia Avenue and Heathfield Road, according to Lt. Hancock.

When the officer got out to approach the vehicle, the driver put it into reverse gear and rammed the police car, then drove off at high speed, Hancock said. The police officer got back into his car and pursued for about a mile until the Jeep collided head-on with a second police car in the 4000 block of Southend Road, police said.

One officer in that police car suffered a knee injury, while a second in the car suffered a head injury. Neither injury was described as requiring hospital treatment.

Flanagan, who police said bumped his head, was questioned at the Wheaton-Glenmont police substation, then taken to Shady Grove Adventist Hospital about 3 1/2 hours after the incident. Hospital personnel said they had been told by police to release no information about him.

Police gave an address on Charlton Court in Silver Spring for Flanagan, but occupants of that residence told a reporter they knew no one by that name.

Flanagan's appearance, noted by onlookers as police took him to a car for the trip to the hospital last night, seems to be at variance with the description of the Aspen Hill assailant that has been circulated by police. He has long blond hair and appears to be more than six feet tall.

A chief hindrance to the police effort to catch the Aspen Hill rapist has been the vagueness with which the assailant has been described. One official remarked at the Tuesday night meeting that the description -- a white man between 18 and 35 with a medium build and possibly brown hair -- "fits about 60 percent of the population." The attacker is said to have worn a ski mask.