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DETROIT (Reuters) - A federal judge entered a not guilty plea on Friday on behalf of Umar Farouk Abdulmutallab, the 23-year-old Nigerian accused in the attempted bombing of a Detroit-bound airplane that prompted a sweeping review of U.S. security procedures.

Abdulmutallab, who shuffled into court in leg irons, answered questions from U.S. Magistrate Mark Randon, who entered the not guilty plea after his court-appointed lawyer said the defendant would "stand mute."

He was arraigned on six charges including attempted murder and the attempted use of a "weapon of mass destruction" to bring down a plane carrying 289 other people.

President Barack Obama took responsibility on Thursday for security failures that allowed Abdulmutallab to board the plane in Amsterdam and ordered reforms aimed at thwarting attacks.

The attempted bombing on Christmas Day prompted a spate of airline security scares that have shut down airports and stranded jittery passengers.

On Friday, Boston's international airport closed for half an hour because of a suspicious smell that officials said may have been de-icing fluid and an AirTran flight from Atlanta to San Francisco was diverted because of an unruly passenger. The U.S. military said two fighter jets were scrambled to escort the plane to Colorado Springs, Colorado.

Officials say Abdulmutallab tried to ignite explosives concealed in his clothing on the flight but was subdued by other passengers fire broke out around his seat.

CBS News, citing British Intelligence, reported on Friday that Abdulmutallab told investigators after his arrest that close to 20 other young Muslim men were being prepared in Yemen to use the same method to blow up airliners.