WASHINGTON — The Federal Aviation Administration created a no-fly zone over the site of two explosions at the annual Boston marathon on Monday, and briefly ordered flights bound for Boston’s Logan International Airport held on the ground at airports around the country.

About an hour after the explosions the FAA issued a notice to pilots that a no-fly zone with a 3.5-mile radius has been created over 811 Boylston Street. The zone was later reduced in a subsequent notice to a 2.3-mile radius. 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 no-fly zone was effective immediately, and will remain in effect until further notice, the agency said. Pilots planning flights are urged to call their local flight service station.

In order to prevent planes from violating the no-fly zone, the FAA briefly held planes bound for Logan around the country on the ground while air traffic procedures at the airport were reconfigured to bring planes in from the northeast and send them out to the southeast over Boston harbor.

Security for outbound international flights at Logan has been stepped up in the wake of the bombings, federal law enforcement officials said. Numerous runners were expected to leave Boston after race, and the additional security has been added as a precaution, they said. The officials requested anonymity because they weren’t authorized to speak publicly.

Transportation officials at airports and train stations around the country also reported stepped up security. In Washington, transit officials said in a Twitter message they knew of no specific or credible threat to the city’s subway system, but “extra officers are on duty for increased security” anyway. It was the same case in New Jersey, where the state’s transit agency said it was in operating “a state of heightened alert.”

“While there has been no specific threat made to our transit system at this time, NJ Transit police have deployed both uniformed and plain clothed officers to further patrol our system and keep our customers safe,” the agency said in a statement.