This undated file photo shows a train in pilot operation on the Donghae inter-Korean railway in 2007. Yonhap

South Korea's spy agency said Wednesday it has detected signs that North Korea has recently been demolishing some sections on the northern side of the inter-Korean railway on the east coast in an apparent move to erase the legacy of inter-Korean exchange and cooperation.

South and North Korea agreed to restore two railways — the Gyeongui and Donghae — in 2000, when the divided countries held the first summit of their leaders. The Donghae railway linked eastern coastal cities across the heavily fortified border.

North Korea has been focusing on erasing unification references since its leader Kim Jong-un defined inter-Korean ties as those "between two states hostile to each other" at a year-end party meeting.

"As there have been recently signs that some parts of the track of the Donghae railway are being demolished, we are monitoring the situation," the National Intelligence Service (NIS) said.

The North has been detected removing a railway sleeper on the northern side of the Donghae line, local newspaper Donga Ilbo reported, citing a Seoul government source.

For the Donghae railway, the track passing through the heavily fortified Demilitarized Zone (DMZ) separating the two Koreas was built in 2006, linking a some 27-kilometer section between the two sides along the east coast. But it has not been operated, except in the pilot operation in 2007.

In recent months, North Korea has been installing mines and removing street lights along its side of two rare roads connecting the two Koreas — the Gyeongui and Donghae land routes — in an apparent bid to completely shut down the routes once regarded as symbols of inter-Korean cooperation and exchange.

Seoul's unification ministry said last week that North Korea may announce "visible" measures to disconnect the Gyeongui route. North Korea is expected to hold a key parliamentary meeting soon to revise the constitution to define South Korea as its "invariable principal enemy" and clarify its territorial boundaries, including the maritime border.

South Korea fully suspended the 2018 inter-Korean military tension reduction agreement Tuesday in response to North Korea's latest trash-carrying balloon campaign and jamming of GPS signals last week.

With the suspension, South Korea said it will restore all military activities near the demarcation line and on its northwestern front-line islands. Seoul will be able to resume field training near the border and could restart loudspeaker propaganda broadcasts against North Korea, a powerful psychological warfare tool. (Yonhap)