By Christine Kearney

NEW YORK (Reuters) - A New York City imam accused of tipping off a suspected al Qaeda-trained militant that he was under investigation pleaded not guilty on Monday to lying to the FBI.

Ahmad Wais Afzali was arrested in September as part of a probe in which federal prosecutors allege the prime suspect, Najibullah Zazi, was planning to set off bombs in the United States.

Attorney General Eric Holder has called the purported Zazi plot one of the most serious security threats to the United States since the attacks of September 11, 2001, saying it could have killed "scores" of Americans had it been successful.

Afzali, the imam, was indicted two weeks ago and faces four counts of making false statements, which carry a combined maximum penalty of 20 years in prison.

Afghan-born Afzali, 37, was arraigned in Brooklyn federal court on Monday and pleaded not guilty to the charges, which include allegations he alerted Zazi that he was under surveillance and then lied about the tip-off when later questioned by the FBI.

Before his arrest, the imam had been used as an informant by New York police. When they asked him about Zazi, also born in Afghanistan, he then alerted Zazi that he was under scrutiny, forcing federal officials to bring Zazi in for questioning sooner than they had planned.

Afzali's lawyer, Ron Kuby, told reporters after Monday's hearing it was implausible his client would lie to the FBI about conversations with Zazi when he knew his conversations were being recorded by the FBI.

"Why on earth would somebody lie about that?" Kuby said. Continued...