Yonhap news agency quoted an official military source as saying that the South Korean military was ready for "immediate action" if North Korea provoked "provocations".
The agency's military source did not specify the quality of the operation, but said at the same time that South Korea and the United States had not yet discovered signs of North Korea's preparations for a new nuclear test or a missile test expected on the eve of the anniversary of the founding of the Korean Workers' Party on 10 October.
South Korea and the United States are monitoring North Korea by all available means, including U-2S and RC-800 reconnaissance aircraft, the RF-16, surveillance and warning aircraft, the Yonhap news agency reported. The early "E-737", and also by P-3C patrols.
In order to monitor North Korea's missiles, within two minutes of its launch, a ship anchored in the Sea of Japan with the Aegis, the SPY-1D and the Green Pine.
South Korean media reported late last month that North Korea had transferred several intercontinental ballistic missiles from a military plant in Pyongyang, which could be a sign of preparation for a hydrogen experiment in the Pacific Ocean.