The North Korean military said it will completely cut off roads and railways linked to South Korea from Wednesday and fortify areas on its side of the border, state media KCNA reported.

The announcement marks a further escalation of activities near the demarcation line between the two Koreas, which was rare in recent years until this year.

North Korea had been planting landmines and barriers and creating wastelands along its heavily militarized border for months this year, despite mishaps, the South Korean military said in July.

The General Staff of the Korean People’s Army said in a statement carried by KCNA that this was in response to war exercises held in South Korea, which it called “the main hostile state and unchangeable main enemy,” as well as to frequent visits by US strategic nuclear assets in the region.

South Korea’s Defense Ministry said in a statement that the United Nations Command (UNC) has been informed of the case, but declined to provide details.

South Korea maintains close communication and cooperation with the UNC on North Korea’s announcement, the ministry added.

The US-led UNC is a multinational military force and oversees affairs in the demilitarized zone (DMZ) between the two Koreas, which are technically still at war.

In addition, North Korea held a session of the Supreme People’s Assembly this week, state media KCNA said on Wednesday, although there was no mention of whether the country had officially changed its position on inter-Korean unification.

Tensions on the Korean Peninsula have increased this year following a series of missile tests and attempts by Pyongyang to change its relationship with the South.

There was widespread expectation that parliament would remove references in the constitution to “unification” between the two Koreas, in line with comments in a speech by leader Kim Jong Un in January.

KCNA also made no mention of any change in the law to designate South Korea as a “primary enemy” or similar language, or any clarification of borders, amid the ongoing tense relationship between the neighbors.