A former State Department arms expert pleaded guilty Friday in federal court to disclosing national defense information to a Fox News reporter.

Stephen Jin-Woo Kim admitted sharing information from a top-secret intelligence report on North Korea with Fox chief Washington correspondent James Rosen. Investigators in the case also targeted Rosen, calling him a possible “co-conspirator” in order to obtain a search warrant for his personal e-mail.

The law enforcement tactics used in Kim’s case — and in another case involving the phone records of Associated Press journalists — led the Justice Department to tighten its policies last summer for pursuing unauthorized disclosures of classified information to journalists.

Kim’s attorneys and federal prosecutors had been engaged in lengthy courtroom battles over documents ahead of a trial, which was scheduled to begin as soon as April.

Kim’s defense team had argued that his conversation with Rosen caused little damage to national security — and that his prosecution could end up causing more harm.

The plea agreement, which must be approved by a judge, calls for Kim to serve 13 months in prison, far less jail time than the maximum possible sentence of 10 years. Prosecutors also have agreed to drop a second charge of lying to federal agents.

“Stephen Kim admits that he wasn’t a whistleblower. He admits that his actions could put America at risk,” U.S. Attorney Ronald C. Machen Jr. said in a statement. “As this prosecution demonstrates, we will not waver in our commitment to pursuing and holding accountable government officials who blatantly disregard their obligations to protect our nation’s most highly guarded secrets.”

Kim, 46, decided to plead guilty, his sister said, because of the “horrific toll” — financially, emotionally and professionally — the case has taken on Kim and his extended family.

“It was the state versus a single individual, an uphill struggle to begin with. He took it on and did the best he could. Now we decide to stop,” Yuri Lustenberger-Kim said in a statement.

The Obama administration has pursued more leak investigations than all previous administrations combined, charging government officials and contractors under the 1917 Espionage Act.

In September, a former FBI bomb technician pleaded guilty to leaking information to the Associated Press about the disruption of a terrorist plot by the Yemen-based al-Qaeda in the Arabian Peninsula.

After Kim’s hearing Friday, his attorney Abbe Lowell said the prosecution underscores problems with the U.S. government’s system for pursuing leaks. The Espionage Act, Lowell said, was designed to punish spies, not to crack down on conversations between government employees and reporters. Lowell suggested that there is a double standard.

“Lower-level employees like Mr. Kim are prosecuted because they are easier targets or often lack the resources or political connections to fight back,” he said. “High-level employees leak classified information to forward their agenda or to make an administration look good with impunity.”

Steven Aftergood, the director of the Project on Government Secrecy at the Federation of American Scientists, said the Espionage Act is such a “blunt instrument” that most accused leakers in recent years have pleaded guilty rather than go to trial.

“It is questionable whether justice is truly served this way,” he said. “Kim had the benefit of a vigorous and aggressive defense, but even that was not enough.”

The investigation into Kim began after Rosen reported in June 2009 that U.S. intelligence officials were warning that North Korea was likely to respond to a new round of United Nations sanctions with more nuclear tests. Rosen reported that the CIA warning was developed through sources inside North Korea.

The story was published online the same day that a classified report was made available to a small circle within the intelligence community — including Kim, who at the time was a senior adviser with a security clearance.

Assistant U.S. Attorney G. Michael Harvey said in court Friday that the news story gave the North Korean government insight into U.S. intelligence sources.

Investigators gathered information from Kim’s office computer and phone records, and obtained a search warrant for Rosen’s personal e-mails.

Edward B. MacMahon Jr., an attorney who is representing a former CIA official in another leak case, said that when the government has evidence of communications, such as e-mails and phone records, it makes it much harder for a defendant to go to trial.

Standing at the lectern of the wood-paneled courtroom on Friday, Kim publicly acknowledged for the first time that he had passed on classified information to Rosen in a brief meeting outside State Department headquarters.

U.S. District Judge Colleen Kollar-Kotelly pointedly asked Kim whether he had “orally disclosed to Mr. Rosen the information” from the intelligence report.

“Yes,” he answered.

Kim is scheduled to be sentenced April 2.