SEOUL, South Korea — The South Korean military went to “crisis status” on Tuesday and threatened military strikes after the North fired dozens of shells at a South Korean island, killing two of the South’s soldiers and setting off an exchange of fire in one of the most serious clashes between the two sides in decades.

President Lee Myung-bak met with security-related ministers and senior aides in the underground situation room at the Blue House, the presidential office and residence, and promised “a strenuous retaliation” if there was any further provocation, said the chief presidential spokesman, Hong Sang-pyo.

The North blamed the South for starting the exchange; the South acknowledged firing test shots in the area but denied that any had fallen in the North’s territory. It was in the same area that a South Korean naval vessel, the Cheonan, was sunk in March, killing 46 sailors. Seoul blamed a North Korean torpedo attack; the North has denied any role.

The United States, Britain and Japan on Tuesday condemned the latest attack. The White House called on North Korea to “halt its belligerent action.” American officials tracking the episode said that a total of 175 artillery shells had been exchanged by the two sides.