The South Korean government said malicious code caused computer networks at several television stations and banks to fail last month.
North Korea was suspected by Seoul of targeting its computer infrastructure in the past years.
"We will cooperate with the U.S. to prepare measures in cyber policy, technology and information," a senior ministry official was quoted by South Korean news agency Yonhap as saying.
Plans for cyberdefense were among the national security agendas presented to South Korean President Park Geun-hye as her administration weighs threats from North Korea.
North Korea pulled out of the armistice that suspended the Korean War in the 1950s and severed the military hotline with Seoul.
The U.S. military last week deployed a B-2 stealth bomber over South Korea airspace amid heightened war rhetoric on the Korean Peninsula.
North Korea billed its February underground nuclear test, its third, as part of a program targeting the United States.
The Internet in North Korea was blocked for about 24 hours starting March 13. The attack allegedly targeted state-run news outlets.