SEOUL, South Korea (AP) — North Korea is preparing to blow up roads that cross the heavily militarized border with South Korea, Seoul said Monday, amid an escalating war of words after the North accused its rival of sending drones over its capital Pyongyang.

North Korean troops were working in camouflage on the roads on their side of the border near the west and east coasts, likely in preparation to blow up the roads, possibly as early as Monday, the South Korean military spokesman said.

North Korea accused South Korea on Friday of sending drones to distribute a “huge number” of anti-North leaflets over Pyongyang. This was a political and military provocation that could lead to an armed conflict.

Lee Sung-jun, a spokesman for the South's Joint Chiefs of Staff, declined Monday to answer questions about whether the South Korean military or civilians flew the drones.

In another harsh statement against South Korea and the United States, Kim Yo Jong, the powerful sister of North Korean leader Kim Jong Un, said on Monday that the South Korean military was “clearly” responsible for the drone attack and that Washington should be held accountable be drawn.

“If the sovereignty of a nuclear-armed state has been violated by mongrels tamed by Yankees, the master of these dogs should be held responsible,” she said via state news agency KCNA, referring to Seoul and Washington.

The North Korean army said last week it would completely cut off roads and railways linked to South Korea and fortify areas on its side of the border, KCNA reported.

North Korea warned at the weekend of a “terrible catastrophe” if South Korean drones flew over Pyongyang again. On Sunday it was said that eight fully armed artillery units were on standby at the border to open fire.

The South Korean military has said its refusal to answer questions about the drones was because confronting the North's claims would lead to Pyongyang being drawn into a tactic to make excuses for provocations.

South Korea has been trying to strengthen its drone defenses since 2022, Lee said, when five North Korean drones entered its airspace and flew over the capital Seoul for several hours.

Lee Kyoung-haing, an expert in military drone operations at Jungwon University, said civilians had no problem obtaining drones with a range of about 186 miles, round-trip from the South to Pyongyang, carrying light payloads such as leaflets.

Other experts say that even if civilians had sent such drones across the border from the south, it may have been difficult to do so without government permission, or it is possible that authorities would have failed to detect and block them.

On Sunday, North Korea's defense ministry said the drones it said were spotted over Pyongyang on three days earlier this month were a special missile launcher or runway and that it would be impossible for a civilian group to use them to fire.

The two Koreas are technically still at war, after their 1950-53 war ended in an armistice rather than a peace treaty.

The cross-border ties are remnants of times of rapprochement between the countries, including a summit between the leaders in 2018 where they declared that there would be no more war and a new era of peace had begun.

North Korea has reintroduced heavy weapons to the Demilitarized Zone's border buffer and restored guard posts after the sides declared a 2018 military agreement on détente was no longer valid.