The three -- who ran a weapons contracting agency after retirement -- are accused of handing over secret information since 2004 while arranging various deals for the US firm, the Seoul Supreme Prosecutors' Office said.

The three, who were not identified, received 2.5 billion won ($2.3 million) in commission fees from the company in 2009 and 2010, the office said in a statement.

It said they obtained information from former colleagues on the air force's plans for weapons purchases and deployments and on budgets.

The leaked information includes the military's plans to purchase Joint Air to Surface Standoff Missiles estimated at some 200 billion won and precision-guided cluster bombs, said a prosecutor on the case.

"Leakage of such information can weaken the military's overall weapons capability against enemies and cause tension with neighbouring countries," the prosecutor, who declined to be identified, told AFP.

Lockheed Martin officials said they did not know the information they had been given was a military secret, making it hard to build a case against the US firm, according to the prosecutor.

"The three veterans are denying the charges...they will likely end up getting jail terms with suspension if convicted," he said.

The veterans have claimed that such information is readily available on the Internet and is not a military secret, according to Yonhap news agency.

First vice foreign minister Kim Kye-Gwan held talks in New York with US officials led by the US special envoy on North Korea, Stephen Bosworth, last Thursday and Friday in the first high-level bilateral contact since 2009.

"I am satisfied with talks this time... (and) will continue the dialogues down the road," South Korea's Yonhap news agency quoted him as saying in New York before leaving for Beijing en route to Pyongyang.

Kim called for "more bilateral talks" before resuming the six-party forum on North Korea's nuclear programme which has been deadlocked for more than two years, Yonhap said.

The North's foreign ministry said on Monday the New York talks were "sincere and constructive" and that Pyongyang and Washington had agreed to hold further dialogues.

Kim also reiterated the communist state's claim that its controversial uranium enrichment programme disclosed last November was a peaceful energy project, Yonhap said.

Experts say it could easily be reconfigured to produce weapons-grade uranium to augment the country's plutonium stockpile.

The six-party nuclear disarmament talks on the North, grouping two Koreas, US, Japan, China and Russia, have been at a standstill since last meeting in December 2008.

Pyongyang stormed out of negotiations in April 2009 and conducted its second nuclear test a month later.

The communist country has repeatedly expressed a desire to go back, but US urged it to show more sincerity and mend ties with the South first.