South Korea's defense ministry reaffirmed its commitment Monday to work with its American counterpart to take back wartime operational control in 2015 as scheduled, despite growing calls to delay the process to counter North Korea aggression.

Asked about the need to postpone the deadline in the wake of growing threats posed by Pyongyang, Defense Minister Kim Kwan-jin told lawmakers that preparations are "well underway" as agreed by the governments of the two nations.

"Currently, the defense ministry sticks to the principle of following the agreement by South Korea and the U.S.," Kim said during a parliamentary defense committee meeting.

As the two sides are required conduct reviews three times before the actual transfer, Kim promised to review the needs to reschedule the process depending on the security situation in the future. 

Regardless of the shift in the wartime control, Kim stressed that the South Korean military should enhance its combat capabilities.

His remarks come as calls have grown for delaying the planned OPCON transition as tensions are running high on the Korean Peninsula after Pyongyang, which conducted its third nuclear test in February, repeatedly threatened nuclear strikes against the South and the U.S. in the past weeks.

In a letter published by local media on Monday, Ret. Gen.

Burwell Bell, who served as commander of the U.S. Forces in South Korea from 2006-2008, said the OPCON transition should be "permanently" postponed as long as North Korea possesses nuclear weapons. The former commander said the communist nation's recent behavior is "unacceptable, highly provocative and very dangerous."

Although the defense chief said Bell's letter reflects the former commander's "personal opinion", he pledged to push for the current process, taking those demands into consideration.

The spokesman for the defense ministry echoed the minister's remarks, saying the OPCON transfer is a "government-to-government agreement" encompassing political and diplomatic issues and military officials of the two nations are "doing their best" to fulfill the goal.

"Leaders of South Korea and the U.S. reached an agreement for the OPCON transfer based on the bilateral alliance and trust,"

defense ministry spokesman Kim Min-seok said in a briefing. "As the two sides had agreed over nuclear umbrella and deterrence strategies, we will continue to cooperate to deal with North Korea's nuclear threat under the agreement."

The spokesman, however, declined to comment on detailed nuclear deterrence plans, which are classified as confidential.

Currently, the South Korean military remains in command under normal armistice circumstances, but Gen. James Thurman, as the commander of U.S. Forces Korea and United Nations forces, would assume operational control if war broke out.

Seoul and Washington have been working to make a new command structure to maintain a combined posture even after the current joint body is dissolved after the transfer.

Under the new structure, the South Korean military will play a leading role, and American forces will support their role during a potential wartime situation, fighting side by side their Korean allies.

In the run-up to the scheduled transfer, Seoul has been stepping up its combat capability with an advanced missile defense system and longer ballistic missiles as the South Korean forces are supposed to play a leading role under the new command structure.

The two Koreas remain technically at war as the 1950-53 Korean War ended in an armistice, rather than a peace treaty. About 28,500 U.S. troops are stationed in South Korea. (Yonhap News)