(This version of the April 20 story, corrects spelling of museum name to Isabella Stewart Gardner Museum in second paragraph)

By Richard Weizel

MILFORD, Conn. (Reuters) - An reputed mobster arrested last week on gun charges told an undercover FBI agent he had access to world-renowned masterpieces stolen 25 years ago in the biggest art heist in U.S. history, a federal prosecutor said on Monday.

That allegation in U.S. District Court in Hartford on Monday came days after the Federal Bureau of Investigation arrested Robert Gentile, 79, for selling a loaded firearm to a convicted killer - an arrest his lawyer said was a ruse to “pressure Gentile” into leading authorities to the long-sought paintings stolen from Boston's Isabella Stewart Gardner Museum in 1990.

The brazen theft of $500 million in artwork at the private art gallery by two men dressed in police uniforms has been the subject of numerous books. None of the 13 paintings, which include Rembrandt's "Storm on the Sea of Galilee" and Vermeer's "The Concert," has been recovered.

At a court hearing on the gun charge, federal prosecutor John Durham said Gentile was secretly recorded claiming to an undercover FBI agent that he had access to at least two of the 13 paintings and was able to sell them for $500,000 apiece.

Gentile appeared in court in a wheelchair and was ordered held without bail by U.S. Magistrate Judge Thomas Smith, who cited his long criminal history and called him "a danger to the society in which we live.”

Defense attorney Ryan McGuigan noted the gun charge carries a maximum penalty of 10 years in prison for his client, since he is a convicted felon, and prosecutors' real aim is to see what he knows about the art theft.

“It is very obvious that is the motive for this arrest and prosecution,” McGuigan said.

Gentile has repeatedly denied knowing the whereabouts of the paintings, his lawyer said.

“As far being a flight risk, he's 79 years old, in very bad health, is morbidly obese and drives a 1989 Buick. I certainly don't think he's going anywhere,” McGuigan said.

FBI investigators have long said they believed organized crime groups were behind the heist, one of the longest unsolved crimes in Boston's history.

A 2012, FBI search of Gentile's home turned up a handwritten list of the stolen art pieces, their estimated value on the black market, and police uniforms, court documents showed.

(Editing by Barbara Goldberg and Andre Grenon)