Some experts say President Moon Jae-in's move is largely symbolic, but others say it shows his determination to carry out the September deals despite growing skepticism about whether his engagement policy will eventually lead to North Korea's nuclear disarmament.
Moon "ratified" the deals on Tuesday afternoon, hours after his Cabinet approved them during a regular meeting, his office said in a statement.
The back-to-back endorsements came with no prior parliamentary endorsement. In South Korea, a president is allowed by law to ratify some agreements with North Korea without consent from lawmakers.
At the start of the Cabinet meeting, Moon said in televised remarks that the ratification would help further improve ties with North Korea and accelerate global efforts to achieve the "complete denuclearization of the Korean Peninsula."
The main conservative opposition Liberty Korea Party criticized Moon's action, saying the deals would only undermine national security and waste taxpayers' money.
"We deplore the fact that the Moon Jae-in government is weighted toward its subservient North Korea policy and is consistently being self-righteous and lacking communication" with parliament, said party spokesman Yoon Young-seok.
Moon, who took office last year, has said that greater reconciliation with North Korea would help resolve the international standoff over the North's nuclear ambitions. Moon has met with Kim three times this year, and he shuttled between Pyongyang and Washington to help arrange a series of high-level talks between the countries, including a June summit between Kim and President Donald Trump in Singapore.
Since entering nuclear talks earlier this year, Kim has taken some steps like dismantling his nuclear testing site and releasing American detainees. The United States responded by suspending some of its annual military drills with South Korea but is reluctant to provide the North with major political or economic rewards unless the country takes significant disarmament steps.
Moon's September deals with Kim were largely associated with the broader agreements struck during their first summit in April. Under the latest deals, the two Koreas are to hold a groundbreaking ceremony on a project to reconnect cross-border railways and roads and push to resume stalled economic cooperation projects. The two sides also agreed to disarm their shared border village, establish buffer zones along the border and withdraw some of their front-line guard posts.
Moon has previously pushed to get parliamentary approval on the April agreements. But conservative lawmakers objected, saying the deals, which had Kim's vague commitment to denuclearization, would only help the North buy time and prefect weapons systems in the face of international sanctions.
Tuesday's ratification follows a contentious ruling by Moon's ministry of government legislation that allowed him to skip parliamentary endorsement on the North Korea accords before ratifying them.
According to the ministry, Moon can unilaterally ratify the deals because they are largely meant to implement the earlier April accords that it says are in the process of getting parliamentary approval. It also cited a law clause that a president can ratify deals with North Korea without lawmakers' approval if they don't cause unspecified "significant" financial burdens to the public or require related legislation.
The opposition party disagreed, saying inter-Korean projects stipulated in the September accords would eventually require "tremendous" taxpayers' money. It also said the deals' mutual reductions of conventional military strength would weaken the South's war readiness and its alliance with the United States because the North's nuclear capability remains intact.
Moon knows how important public support is for his North Korea overture. Most of the detente projects mentioned in his summit deals with Kim were what his liberal predecessors had pursued during a 1998-2008 "Sunshine Era." Those projects were stalled after conservatives took power in South Korea. Moon now cannot unilaterally revive those projects because of U.S.-led international sanctions.