SEOUL, South Korea  The North Korean military declared an “all-out confrontational posture” against South Korea on Saturday as an American scholar said North Korean officials told him they had “weaponized” enough plutonium for roughly four or five nuclear bombs.

American intelligence officials have previously estimated that the North had harvested enough fuel for six or more bombs, although it has never been clear whether the North constructed the weapons. The scholar, Selig S. Harrison, said the officials had not defined what “weaponized” meant, but the implication was that they had built nuclear arms.

The North conducted a test of a nuclear device in 2006, but it appeared to result in a fizzle and experts concluded the explosion was relatively small. While the country has often claimed to possess a “deterrent,” this appears to be the first time it has quantified how much plutonium it says it has turned into weapons.

After the threats on Saturday, South Korea ordered its military to heighten vigilance along the heavily fortified border with North Korea, according to a spokesman for the South Korean military joint chiefs of staff.