TORONTO  In its strongest move since the sinking of a South Korean warship, the Obama administration said Saturday that the United States would retain control of all military forces in the South during any conflict with North Korea, which has been widely blamed for the attack on the ship in March that killed 46 sailors.

The announcement was an apparent attempt to signal to the North, which has long wanted American forces off the peninsula, that the United States would remain firmly in control of military operations if war were to break out.

The decision is somewhat symbolic; the United States was not slated to give up wartime control of South Korean troops until 2012, and the new agreement extends the deadline to 2015. But the agreement allowed Washington and Seoul to take some action after months of struggling for ways to punish the North  and attempt to deter it from further violence  without provoking the country’s erratic leader, Kim Jong-il, to launch new attacks.

“There have to be consequences for such irresponsible behavior on the international stage,” Mr. Obama said Saturday during a press conference with President Lee Myung-bak of South Korea on the sidelines of an economic summit.