Dozens of North Korean submarines that supposedly disappeared off South Korea's military radar on Monday have now apparently started returning to their bases, according to a military official quoted by Asian news sources on Tuesday.

South Korea said on Sunday that more than 50 of North Korea's submarines, about 70 percent of the country's known fleet, had left their home ports and could not be located, heightening fears that Pyongyang might be poised to attack.

But following a truce made between the two Koreas in the early hours of Tuesday morning to end an escalating stand-off, a South Korean defense ministry official was reported as saying the missing submarines had been detected in North Korea's inland sea.

"Some 50 submarines that had been away from their bases since August 21 have shown signs of returning back to their bases," said the official, quoted by various South Asian media.

Defense analyst Moon Geun-Shik told South Korean newspaper Chosun Ilbo that it was a deliberate show of strength by North Korea to place pressure on the South during talks between the two sides.

Officials cited by the Korea Herald on Tuesday said the submarines may have now returned due to a typhoon that is nearing the Korean Peninsula, or because the vessels can only stay underwater for three days at a time.

As part of the agreement announced in the early hours of Tuesday (local time) after several days of marathon negotiations, South Korea agreed to stop blasting propaganda from loudspeakers pointed at the militarized border with the North — a tactic it hadn't employed in 11 years — while Pyongyang said it regretted a land mine explosion earlier this month that injured two South Korean soldiers. Further details of the talks were not disclosed.

North Korea called the blaring propaganda an "act of war," and criticized a joint military exercise between the US and South Korea.

The announcement of a truce comes after the two Koreas exchanged live artillery fire between the border last Thursday and Friday, which South Korea said was started by the North. It was the first time that North Korea directly attacked South Korean land since 2010, when they shelled the island of Yeonpyeong, killing four people.

Also on Tuesday, it was announced China and North Korea would open a border trade zone in October.