South Korean President Park Geun-hye has called for tighter vigilance against North Korea, days after its leader Kim Jong-un warned of a "very grave situation" on the peninsula.
At a meeting yesterday with top aides, Park noted that North Korea had recently threatened to fire off a fresh nuclear test, tested missiles and lobbed artillery shells into the waters across the sea border.
Three drones suspected to have flown from the North to scout South Korea's military facilities were also found in the South over the past month.
"We have to take it seriously that North Korea has been continuously firing missiles and intensifying reconnoitring," Park was quoted as saying. "We must prepare ourselves for possible further provocative acts by North Korea and take measures to stop and repel such provocative acts."
Park said the infiltration by drones had exposed "loopholes in our anti-air defence and on-ground surveillance system".
"Counter measures must be swiftly taken and the defence of key facilities must also be bolstered."
Defence Minister Kim Kwan-jin said the North would be able to develop drones for terrorism.
"If [North Korea] developed the small unmanned aerial aircraft for reconnaissance purposes to enhance its relatively weak surveillance capability, it is expected to develop drones for infiltration and terrorism purposes in the future," Kim said during a video conference at the joint chiefs of staff headquarters in Seoul.
"We should strengthen our military readiness to be able to monitor, detect, identify and strike [the drones] with existing military assets along the border", he was quoted as saying by Yonhap.
The defence ministry yesterday ordered the military to search for other drones that may have crashed, a spokesman said.
Kim Jong-un said last week that a "very grave" situation had been allowed to develop on the Korean peninsula.
In a meeting with his top military leaders, Kim blamed the United States and Seoul for the frictions, saying they had trampled on peace overtures from Pyongyang.
He said that despite the North's conciliatory gestures, South Korea and the US had pushed ahead with joint military drills that Pyongyang viewed as rehearsals for invasion.
The annual military exercises, which began in February, are to end on Friday next week.