(Reuters) - Five bomb threats that turned out to be hoaxes disrupted U.S. domestic and international flights on Tuesday, airline officials and media reports said.

Police met U.S. Airways flight 648, with 88 passengers and five crew on board, when it landed at Philadelphia International Airport due to a "possible security threat," said Victoria Lupica, a spokeswoman for U.S. Airways' parent company, American Airlines Group Inc

A possible bomb threat prompted the search, Philadelphia's 6ABC television reported. No explosives were found.

NBC and CNN reported that other U.S. commercial flights received bomb threats on Tuesday: a United Airlines [UALCO.UL] flight to Chicago, Delta Air Lines flight to Atlanta and a Volaris flight <AVT_p.CN> from Portland to Guadalajara, Mexico. The planes landed safely and were searched, and passengers were deplaned, according to CNN.

United spokesman Charles Hobart said in a statement that authorities cleared its Flight 955 upon arrival in Chicago. "Passengers exited the plane as usual," he said. "We are working with law enforcement in their review."

A fifth flight operated by Korean Air Lines and bound for San Francisco was still in the air, NBC reported.

Officials with the Federal Bureau of Investigation were not immediately available for comment. The Transportation Security Administration and Federal Aviation Administration had referred queries to the FBI.

On Monday, Homeland Security Secretary Jeh Johnson reassigned the acting administrator for the Transportation Security Administration after ordering improved security at U.S. airports.

Monday's news followed media reports that checkpoint screeners did not detect mock explosives and weapons in 95 percent of tests carried out by undercover agents.

Separately, some 188 flights on United, or about 9 percent of its total flights, were delayed on Tuesday, according to flight tracker FlightAware.com.

The FAA halted the airline's flights for 39 minutes because of what it described as automation issues. Hobart said United began delaying flights at about 9 a.m. EDT "to ensure aircraft departed with proper dispatching" and resumed departures a short time later.

(Reporting by Suzannah Gonzales in Chicago and Jeffrey Dastin in Los Angeles; Additional reporting by Doina Chiacu in Washington; Editing by Scott Malone, Doina Chiacu and Cynthia Osterman)