WASHINGTON - The United States could fight an old-fashioned war against North Korea if necessary, even while newer forms of conflict against terrorists and extremists continue, the Army's top officer said Thursday.

Asked whether the United States would be prepared to fight if war broke out between South Korea and North Korea, Gen. George Casey replied, "The short answer is yes," then added that "it would probably take us a little bit longer to shift gears" away from the type of counterinsurgency fighting that now occupies the Army.

Casey said his usual rubric for how long it would take the Army to gear up for a new "conventional" war is about 90 days. That doesn't mean it would take 90 days for the U.S. to effectively fight the North's million-man army, he said.

"We'd move forces as rapidly as we could get them prepared," Casey said during an appearance at the Center for Strategic and International Studies.

North Korea has threatened war following condemnation of its underground nuclear test this week, and the United States has a long-term commitment to South Korea's defense.

"This is a combat-seasoned force" that can pivot quickly, Casey said.

Defense Secretary Robert Gates, speaking to reporters as he traveled to the Far East for a conference with defense ministers, said North Korea's actions have not reached a crisis level that would warrant additional U.S. troops in the region.

"What we do have, though, are two new developments that are very provocative, that are aggressive, accompanied by very aggressive rhetoric," Gates said. "And I think it brings home the reality of the challenge that North Korea poses to the region and to the international community."

Casey, the Army's chief of staff, suggested that war with the nuclear-armed North might not be the old-style land war that U.S. forces stationed in South Korea were envisioned to fight. He did not elaborate, but he was presumably referring to the possibility that the North might use or threaten to use its proven nuclear capability.

Casey focused on his plans to rearrange the Army around the "reality scenario" of sustained counterterrorism conflict. The reality of permanent war means the United States should have 10 Army brigades and Marine Corps regiments available for overseas conflict worldwide, he said.

"It's not just Iraq and Afghanistan," Casey said. Including Iraq in his contingency planning is not to say that the United States won't honor its agreement with Iraq to pull forces from the country by 2012, he said.

"We will execute the draw down plan that has been executed between our governments," he said.

"I don't know that anyone knows what the security relationship and force level will be, if there are any, in Iraq," after the scheduled withdrawal of combat forces," he added. "That's very much to be determined."