North Korea ratcheted up its flak against South Korea Thursday over the opening in Seoul of a U.N. office on the North's human rights, warning of "catastrophic" consequences in bilateral ties.

The U.N. High Commissioner for Human Rights on Tuesday opened a field office in Seoul aimed at monitoring North Korea's human rights.

North Korea has threatened to retaliate against South Korea, saying it will "mercilessly punish" South Korea by mobilizing all means possible.

North Korea raised the level of its criticism against the South, saying that the establishment of the office is a "blatant challenge" to the North's dignity and social system and an "open declaration of war" against Pyongyang.

"(President) Park Geun-hye will have to entirely take the blame for all the ensuing catastrophic consequences in the North-South relations from this moment," the North's Committee for the Peaceful Reunification said in a statement.

It said that there is no more need to deal with those who are keen on "hysteric" confrontation and what remains to be done is only the "final showdown."

In response, a Unification Ministry official said that it is "deplorable" that the North has threatened to retaliate the South.

"North Korea should not respond to the global community's concerns with criticism and threats. Pyongyang should make efforts to improve the human rights of its people," said the official, asking not to be named.

North Korea has long been labeled one of the world's worst human rights violators, but it has bristled at such criticism, calling it a U.S.-led attempt to topple its regime.

In what could be seen as the North's anger against the U.N. move, the North on Tuesday sentenced two South Koreans -- Kim Kuk-gi and Choe Chun-gil -- to hard labor for life for spying for the South's intelligence agency.

The North's move is likely to dampen inter-Korean ties which have been strained amid the North's consistent pursuit of nuclear weapons and ballistic missiles.

Last week, North Korea offered conditional proposals for talks with South Korea, calling on Seoul to meet some preconditions for dialogue, such as the suspension of joint military drills with the United States. Seoul voiced regret over Pyongyang's move to set preconditions for talks. (Yonhap)