South Korean President Park Geun-Hye called Monday 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 with top aides, Park noted North Korea had recently threatened a fresh nuclear test, test-fired missiles and lobbed artillery shells 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 by a media pool report.
"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".
"Countermeasures 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 news agency.
The defence ministry on Monday ordered military units across the country to search for other drones that may have crashed, a spokesman said.
The North's young leader Kim said last week a "very grave" situation existed on the Korean peninsula.
Military tensions have risen in recent weeks, with a spate of missile and rocket tests by the North. The two sides have also traded artillery fire into the sea across their border and Pyongyang has threatened a new nuclear test.
In a meeting with his top military leaders, Kim blamed Washington and Seoul for the current frictions, saying they had trampled on peace overtures from Pyongyang.
He said that despite the North's conciliatory gestures, South Korea and the United States had pushed ahead with joint military drills that Pyongyang views as rehearsals for an invasion.
The annual military exercises, which began in February, are to end on Friday next week.
On March 31 North Korea conducted a live-fire artillery drill along the disputed sea border with the South.
After some shells crossed the line, South Korea responded and the two sides fired hundreds of artillery rounds into each other's territorial waters.
The exchange of fire came a day after North Korea warned that it might carry out a "new" type of nuclear test — a possible reference to a uranium-based device or a miniaturised warhead small enough to fit on a ballistic missile.
Although South Korean intelligence reports say there are no signs of an imminent test, analysts note that the North is treading a similar path to that preceding its earlier tests.
