The Federal Aviation Administration is warning pilots that it has created a no-fly zone over the site of two explosions at the annual Boston marathon.

The agency said in a notice issued Monday about an hour after the explosions that a no-fly zone with a 3.5-mile radius has been created over 811 Boylston Street. The zone is limited to flights under 3,000 feet in altitude, which is lower than most airliners would fly except when taking off or landing.

The notice says the no-fly zone is effective immediately, and will remain in effect until further notice. Pilots planning flights were urged to call their local flight service station.

Cellphone service at a crawl

Cellphone companies say service is operating in the Boston area, but with heavy traffic following of the explosions at the Boston Marathon.

A law enforcement official, citing an intelligence briefing, said cellphone service had been shut down Monday in the Boston area to prevent any potential remote detonations of explosives.

But officials with Verizon Wireless and Sprint Nextel said there had been no such requests.

Sprint spokeswoman Crystal Davis said: "Minus some mild call blocking on our Boston network due to increased traffic, our service is operating normally."

Three people were killed and scores injured when two explosives detonated near the finish line of the marathon.