"The security situation on the Korean Peninsula for now is very grave as the unpredictability surrounding North Korea is rising following its third nuclear test," Foreign Minister Yun Byung-se said in a Yonhap news agency report.
"However, my aim is to turn this era of confrontation and mistrust into an era of trust and cooperation with North Korea," Yun said shortly after his appointment by South Korean President Park Geun-hye, who appointed 13 ministers at her first Cabinet meeting.
North Korea ramped up its rhetoric after the U.N. Security Council adopted a tougher resolution against the Pyongyang for conducting its third nuclear test last month.
Yun, a career diplomat with expertise on North Korean nuclear issues, made his comments as South Korea and the United States begin annual military exercises that sparked the ire of Pyongyang.
Yun, 60, came to the attention of Park in 2010 when he was a member of the Institute for the Nation's Future, a think tank founded by Park, a report by The Seoul Times newspaper said.
Yun has served at South Korea's mission in Geneva and at the South Korean Embassy in Washington.
He spent nearly a decade focusing on South Korea's relations with the United States and dealing with North Korea policy including nuclear issues., the Seoul Times report.
Yun has a master's degree from Johns Hopkins University's Paul Nitze School of Advanced International Studies and is a visiting professor at Sogang University's Graduate School of International Studies.
The appointments of Yun and 12 other ministers are the first for Park who took office Feb. 25 after winning a general election Dec. 19, taking nearly 52 percent of the vote.
She is expected to appoint five more Cabinet members, including a defense minister, this week.
Park, 60, is the daughter of former President Park Chung-hee, a former junta general who seized power in a military coup in 1961. He was elected president in 1963, a post he held until he was assassinated by the chief of his own security services in October 1979.
Her mother, Yook Young-su, was killed in an assassination attempt on her father by a pro-North Korean man in 1974.
During her election campaign Park put a high priority on national security but accompanied with improved dialogue with the North.
After her Cabinet appointments this week Park reiterated the need to improve diplomatic channels with the North.
"If we are going to get North Korea to give up its nuclear programs and make the right choice, what is more important than anything else is to cooperate closely with the international community," she said.
"I hope the (foreign) ministry will actively mobilize diplomatic channels and come up with countermeasures," she said.
A spokesman for Park said she also instructed the unification ministry to ensure the safety of South Korean workers at a joint industrial complex in the North Korean city of Kaesong and residents on the border island of Yeonpyeong.
Tensions between North Korea and South -- still technically at war since 1953 when the peninsula was divided -- have been high since North Korea's rocket launch in December which was condemned as a violation of U.N. resolutions.
Pyongyang said the rocket carried an observation satellite into orbit.
Last week a commentary in a North Korean government newspaper threatened the United States with "real war" if it goes ahead with military exercises with South Korea.
The "Key Resolve" exercise involves more than 13,000 troops and another joint exercise, Foal Eagle, has been under way since the beginning of March.
In retaliation, Pyongyang threatened to cut the emergency hotline with the South and cancel two non-aggression agreements.
Yopnhap reported that the North cut off a hotline set up at the truce village of Panmunjom on the inter-Korean border to prevent accidental clashes between the sides.