Kim Jong Un says he will react with nuclear weapons to enemy threats 0:46
(CNN) --
Five North Korean drones crossed South Korean airspace on Monday, forcing the South Korean military to deploy fighter jets and attack helicopters, the country's Defense Ministry said.
The South Korean military fired on the drones but could not confirm if any were downed.
Lee Seung-oh, a South Korean defense official, said four of the drones flew around Ganghwa Island and one flew over the airspace north of the capital Seoul.
Taiwan says 47 Chinese planes crossed Taiwan Strait amid new Chinese military exercises
"This is a clear provocation and an invasion of our airspace by North Korea," Lee said during a briefing.
In response to the airspace violation, the South Korean military sent its manned and unmanned reconnaissance assets to the inter-Korean border region, and some of them crossed into North Korean territory.
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The teams carried out a reconnaissance mission, which included filming North Korean military installations, Lee added.
The South Korean military first detected the drone in the skies near the northwestern city of Gimpo at around 10:25 a.m. local time Monday, according to the country's Defense Ministry.
Kim Jong Un says he will react with nuclear weapons to enemy threats 0:46
The last time a North Korean drone was detected under the inter-Korean border was in 2017, according to the South Korean Defense Ministry.
At the time, South Korea said it had recovered a crashed North Korean drone that was spying on a US-built missile system in the country.
North Korea has aggressively stepped up its missile tests this year, often launching multiple weapons at once.
It has fired missiles on 36 separate days, the highest annual number since Kim Jong Un took power in 2012.
Most recently, North Korea launched two short-range ballistic missiles on Friday, according to South Korean officials.
The missiles were fired from Pyongyang's Sunan area into the waters between the Korean peninsula and Japan.
North Korea fires short-range ballistic missiles, South Korea says
The secretive country often tests its missiles this way, firing them at an oblique angle so they land in the waters between the Korean peninsula and Japan.
However, in October it fired an intermediate-range ballistic missile (IRBM) on a normal trajectory that passed over Japan for the first time in five years.
In November, he claimed to have launched a "new type" of intercontinental ballistic missile, the Hwasong-17, from Pyongyang International Airfield, a missile that could theoretically reach the continental United States.
And last week, Kim Yo Jong, the sister of Kim Jong Un and a senior regime official, claimed on state media that North Korea was prepared to test firing an intercontinental ballistic missile (ICBM) with a normal trajectory, a pattern of flight that could show that the weapons could threaten the continental United States.
The United States and South Korean experts have warned that Pyongyang may be preparing a nuclear test, the first in more than five years.
North Korea has been developing its nuclear missile forces in violation of UN Security Council resolutions, intensifying its activities since the last of three 2019 meetings between Kim Jong Un and then-US President Donald Trump did not reach an agreement.
In October, Kim warned that his nuclear forces are fully prepared for a "real war."
"Our nuclear combat forces... once again demonstrated their full readiness for real war to bring enemies under their control," Kim said in comments reported by North Korea's state-run Central News Agency.
CNN's Yoonjung Seo, Brad Lendon, KJ Kwon, Joshua Berlinger and Emiko Jozuka contributed to this article.
North Korea South Korea