Also on Thursday, Ms. Park appointed Ahn Dae-hee, a former Supreme Court justice who helped her election campaign in 2012, to replace Chung Hong-won as the prime minister, the largely ceremonial No. 2 post in the government. Mr. Chung offered to resign on April 27 with an apology over the ferry sinking.

Mr. Ahn’s appointment is subject to parliamentary approval.

Mr. Nam and Mr. Kim, both former army generals, have served as Ms. Park’s top policy advisers on North Korea since she took office in early 2012. They helped lead South Korea’s hard-line and confrontational stance against the North as Pyongyang escalated tensions on the divided Korean Peninsula following its nuclear test in February 2012.

Prosecutors looking into the cause of the ferry’s sinking have said that the ship was overloaded and badly unbalanced and was manned by a crew poorly trained in safety measures and seamanship. Yet, it was allowed to set sail by pier-side inspectors who were not properly supervised by government regulators.

Ms. Park’s government was accused of squandering the critical first hour of the disaster by failing to respond quickly to repeated distress calls from passengers and the crew. Images of the ferry with hundreds of young students trapped inside slowing turning over and sinking while coast guard ships watched have outraged the nation.

Weekly approval ratings of Ms. Park fell for four straight weeks to 51 percent last week, according to Realmeter, a public opinion survey company that interviewed more than 2,500 adult South Koreans from Monday through Friday last week. During a nationally televised speech on Monday, a tearful Ms. Park apologized again.