The Obama administration is planning a long-term program to plug major gaps in the South's naval defenses, senior American officials have revealed.

The sinking of the South Korean warship "Cheonan" by North Korea had still left the country vulnerable to surprise attack, they said.

The discovery of the weaknesses in South Korea caught officials in both countries off guard.

It has been reported that with South Korea ranked among the world's top economies, there is now an urgent need to bolster its defenses with the help of millions of American dollars to repel any attempt of a North Korean invasion.

Firepower sent to the Korean region by warplanes and warships would more than compensate for the drop in American troop levels there in the event of war, The New York Times has quoted senior Pentagon officials as saying.

In an interview last week, Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, said that the joint training exercise with South Korea planned just off the country's coast in the next few weeks represented only the "near-term piece" of a larger strategy to prevent a recurrence of the kind of shock the South experienced as it watched one of its ships sunk without warning.

Admiral Mullen said, "Longer term, it is a skill set that we are going to continue to press on. Clearly, we don't want that to happen again. We don't want to give that option to North Korea in the future. We want to take it away."

The report further noted that American and South Korean officials declined to describe details of the coming joint exercises, except to say that they would focus on practicing antisubmarine warfare techniques and the interdiction of cargo vessels carrying prohibited nuclear materials and banned weapons. (With Inputs from Agencies)