National security adviser Chung Eui-yong told President Moon Jae-in he expected North Korea activity around October 10 and 18.
Park Wan-ju, head spokesman of the ruling Democratic Party, said: “Chung’s report also said there are worries over military conflict being sparked by accidental incidents.
“The president said the United States speaks of military and diplomatic options, but South Korea can’t go through war again.”
Tensions on the Korean peninsula have escalated in recent weeks as North Korea’s leader Kim Jong-un and US President Donald Trump exchange threats and insults over the Pyongyang’s nuclear and missile development program.
North Korea has accused Mr Trump of declaring war after he warned Kim’s regime if he persisted in threatening the US and its allies after telling UN General Assembly members North Korea would be “totally destroyed” in such an event.
Pyongyang conducted its sixth and largest nuclear test on September 3 and has launched dozens of missiles this year as it accelerates a program aimed at enabling it to target the United States with a nuclear-tipped missile.
China would be furious if Kim tested a missile or carried out some other act during its Communist Party Congress, held once every five years.
Beijing has already started to put pressure on the pot-bellied despot by introducing tough trade and economic sanctions in line with a UN directive.
But officials are anxious to prevent a humanitarian crisis in North Korea amid fears it could spill across the border into China.
Mr Moon said cooperation with the international community to curb North Korea’s nuclear ambitions was at its highest level ever and called for the strengthening of South Korea-US defences to rein in the North.
The president said he had personally been against the deployment of US Terminal High Altitude Area Defense (THAAD) system in South Korea, but the decision was made as North Korea’s missile capabilities were quickly improving.
Regarding China’s vehement opposition to the deployment of THAAD, Mr Moon said “visible achievements” were to be expected soon as “the issue was reaching the end”.
Beijing opposes THAAD because it believes its powerful radar could be used to look inside its territory.
South Korea and the US insist it is only to curb North Korea’s missile threats.