Washington - On the night a torpedo-armed North Korean submarine allegedly sank a South Korean patrol ship, the U.S. and South Korea were engaged in joint anti-submarine warfare exercises just 75 miles away, military officials told The Associated Press.

The sinking of the Cheonan was the worst South Korean military disaster since the 1950-53 Korean War. It showed that even impoverished nations such as North Korea can inflict heavy casualties on far better equipped and trained forces, including those backed by U.S. military might.

Defense Secretary Robert Gates said plans for more joint U.S.-South Korea anti-submarine exercises, announced after Cheonan went down, are on hold awaiting U.N. action on the incident.

That's in part, Gates said Friday while in Asia, because of concerns about instigating another rash act by the North Koreans.

Two months after the sinking, U.S. officials for the first time disclosed details of the joint naval exercise held the same day as the attack on the Cheonan. Forty-six South Korean sailors died on the warship, which was not involved in the exercise. It was on routine patrol near disputed waters.

Military officials said the drill could not have detected the North Korean sub. Officials and defense experts said a minisub would have been difficult for even a nearby ship to track in shallow coastal waters.

What surprised experts was that a 130-ton minisub, without warning, could bring down a warship nine or 10 times its size, a power mismatch called asymmetric warfare.

"To us, stealth denotes the latest technology - billions of dollars in research and development in armaments," said John Park, a Korea expert at the U.S. Institute of Peace. "The North Korean version of stealth is old-school diesel-battery operated subs that evade modern detection methods."

A South Korean-led investigation into the sinking concluded last month that the evidence overwhelmingly pointed to the North. The North has denied any involvement.

Western experts say there are still questions about exactly what happened that night off Baengnyeong island.

One U.S. official said the sinking may not have been an intentional attack at all, but the act of a rogue commander, an accident or an exercise gone wrong. He spoke on condition of anonymity because he was not authorized to discuss the incident publicly.

A statement run by North Korean state media threatened war in response to any attempt to punish the North.

"Because of the South Korean war-loving, mad puppets and American invaders, the North and South relationship is being driven to a catastrophe," Choi Yong Rim, a high-ranking North Korean Party official, told a rally last week in the North Korean capital.

On Saturday, South Korea's president, Lee Myung-bak, tried to lower fears of an armed conflict. "There is absolutely no possibility of a full-scale war on the Korean peninsula," he said in Singapore.

U.S. and South Korean forces can easily monitor the movements of North Korean submarines when they operate on the surface.

Underwater, tracking submarines relies on active or passive sonar. Passive sonar uses microphones to listen for the sounds of sub operations. Active sonar emits sounds and listens for the echoes as they bounce off of submerged objects.

The Cheonan was operating its active sonar at the time, South Korean Navy officer Kim Young-kyu told The Associated Press. It wasn't clear why the ship didn't detect the sub.

After the blast, a South Korean commander dispatched a patrol boat to look for subs.

But officials said the vessel couldn't locate any, perhaps because of the weather, currents and rough conditions that chilly March night. Those factors, as well as the rocks and ledges in shallow water, can all affect the reliability of sonar, experts say.

Sonar technology has traditionally been designed to operate in deep waters and used for convoy protection rather than coastal defense.

"There's a lot of equipment that works pretty well against big submarines out in the deep ocean, but doesn't work so well against small submarines in shallow water," said John Pike, director of GlobalSecurity.com, a military think tank. "We've got the same concern with Iran and the Persian Gulf."

North Korea is believed to have a fleet of 70 submarines, including some 50 that are small but still capable of carrying a torpedo.

The night before the Cheonan sank, two U.S. destroyers and other ships maneuvered and practiced tracking while a South Korean navy submarine played the role of target.

The U.S.-South Korean anti-sub exercise began at 10 p.m. March 25 and ended at 9 p.m. the next day, Army Col. Jane Crichton, a spokeswoman for U.S. forces in Korea, told the AP. The exercise was terminated because of the blast aboard the Cheonan.