By Noeleen Walder

NEW YORK (Reuters) - On the afternoon of May 14, moments after authorities pulled Dominique Strauss-Kahn off an Air France flight on suspicion that he had tried to rape a hotel maid, the former International Monetary Fund chief pulled rank.

"I have diplomatic immunity," he told police, and asked to speak with the French consulate, according to court papers.

Hours later, as police questioned him about his diplomatic status, and whether he was claiming immunity, Strauss-Kahn did an about-face.

"No, no, no. I am not trying to use that," he said. "I just want to know if I need a lawyer."

More than four months on, with the criminal case now behind him, Strauss-Kahn is taking another crack at invoking diplomatic immunity -- this time to fend off a civil suit filed by Nafissatou Diallo, the maid who accused him of sexually assaulting her in his suite at the Sofitel Hotel in Manhattan.

In court papers filed on Monday, Strauss-Kahn's lawyers asked a judge to dismiss the civil suit, arguing that as head of the IMF, Strauss-Kahn was entitled to absolute diplomatic immunity, even after he had resigned.

But international law experts and criminal-defense lawyers said convincing a judge to buy the immunity argument will be a formidable task.

"I think (his lawyers) are feeling somewhat emboldened and they are trying anything. But I don't think it has legs," said Bradley Simon, a prominent New York criminal-defense lawyer.

Simon said if the immunity defense had any legitimacy, Strauss-Kahn's lawyers would have raised it early in the criminal case, before he stepped down as head of the IMF. Strauss-Kahn resigned on May 18, saying in a letter to colleagues that he wanted "to protect" the institution.

'THERE IS NO PRACTICE HERE'

Strauss-Kahn's lawyer, William Taylor, who also represented him in the criminal case, wrote in an email that his client "chose not to raise immunity in the criminal case in order to defend against the false charges and to clear his name."

"We have said from the beginning that this matter was an attempt by plaintiff and her lawyers to extort money from Mr. Strauss-Kahn," Taylor wrote.

Prosecutors dropped all charges against Strauss-Kahn on August 23, weeks after acknowledging that they had grave doubts about Diallo's credibility.

In arguing that the court should dismiss Diallo's civil suit, Strauss-Kahn's lawyers cited a 1947 United Nations convention under which heads of specialized agencies are immune from civil and criminal suits, regardless of whether they were acting in their official capacity when the alleged harm took place.

The United States is not a party to the convention but Strauss-Kahn's lawyers insisted that its absolute-immunity provision has "achieved the status of what is known as 'customary international law'" and that it must be honored.

Kurt Taylor Gaubatz, an associate professor in the Graduate Program in International Studies at Old Dominion University, called this argument a stretch.

The convention is a "relatively obscure treaty that hasn't been tested," Gaubatz said, adding that he was not aware of any other accusations of criminal acts by agency directors. "Customary law develops through practice. And there is no practice here."

'DEATH-BLOW TO THE CASE'

Chimene Keitner, a law professor at the University of California in Hastings, agreed that Strauss-Kahn had a weak argument, noting that neither the IMF nor the State Department asserted the immunity claim on Strauss-Kahn's behalf after his arrest.

"Customary international law is something that so many states do that we can basically presume that they all consent to, even if they haven't signed any documents," she said. "That's a pretty high bar."

But Paul Callan, a New York trial lawyer and professor of media law at Seton Hall University, said a motion to dismiss is based strictly on legal arguments and not evidence that may come out during the discovery process of a trial.

In that light, Strauss-Kahn's "aggressive approach" makes sense, since "there's always a chance that a sympathetic judge will toss the entire case," Callan said. In the meantime, he said, Strauss-Kahn had nothing to lose by taking this tack.

"It would deal a death-blow to the case before embarrassing depositions and discovery proceedings are required," he said.

Douglas Wigdor, one of Diallo's lawyers, wrote in an email that the motion to dismiss was "nothing more than another attempt to avoid having to answer for the deplorable acts" Strauss-Kahn is accused of committing by Diallo.

NOT A DIPLOMAT

"Mr. Strauss-Kahn did not invoke diplomatic immunity in the criminal case because he was not a diplomat under any applicable treaty or law," Wigdor wrote. "He was on 'personal' business at the time and was acting in his personal capacity when he attacked Ms. Diallo. The IMF, the United States State Department and the New York Police Department all agree that Mr. Strauss-Kahn lacks immunity."

Even if Strauss-Kahn is able to persuade a judge that international law entitles him to immunity, legal experts say he still could lose the argument, since it is the IMF's decision whether to support an immunity claim for one of its employees.

"The immunity attaches as the result of the work (he's) doing for the organization or the country, and the organization or the country retains the right to waive that immunity," said Paul Cohen, an international law expert with Holland & Knight

A spokesman for the IMF declined to comment on whether it would waive diplomatic immunity for Strauss-Kahn.

On September 17, Strauss-Kahn, who returned to France after the charges against him were dropped, gave an interview on French television in which he said the encounter with Diallo was consensual, but added, "It was a moral error and I am not proud of it."

(Reporting by Noeleen Walder; Additional reporting by Joseph Ax; Editing by David Storey)