Describing what the confrontation might look like, Gen. Curtis Scaparrrotti said that, "Given the size of the forces and the the weaponry involved, this would be more akin to the Korean War and World War II -- very complex, probably high casualty."
Scaparrotti made the statement while testifying before the House Armed Services Committee.
The U.S. military suffered 405,399 fatalities in World War II and 36,574 during the Korean War of 1950-1953. Korean casualties were in the millions.
Scaparrotti said Tuesday that North Korean leader Kim Jong Un would use a weapon of mass destruction if he thought the fate of his rule was at stake.
If he "thought his regime were challenged he states that he would use WMD," Scaparrotti told a Senate Armed Services Committee hearing.
He said that tensions on the Korean Peninsula were at their highest level in more than 20 years.
Adm. Harry Harris, commander of U.S. Pacific Command, said at the same hearing that if North Korea continues to develop ballistic missiles, a U.S. military option is possible.
The North Korean military warned the U.S. and South Korea Tuesday to expect retaliations for their annual joint military drill in March. The warning came in a statement released Tuesday via state media agency KCNA.
The U.S. maintains nearly 30,000 troops on the Korean Peninsula. These troops operate in alliance with the 655,000-strong South Korean armed forces, while North Korea fields a military of 1.19 million service members, according to a tabulation by the International Institute for Strategic Studies.
North Korea's recent nuclear and long-range missile tests have prompted formal discussions on the deployment of the THAAD missile defense system, which can target short, medium and intermediate ballistic missiles in flight.
China has been outspoken in opposing deployment of the system, which it is worried could be used against its own launch systems.
Harris called China's opposition to missile defense aimed at North Korea "preposterous."
Harris also accused China of aggressive behavior in the South China Sea, noting its building of artificial islands and deployment of anti-aircraft missiles and radar to islands in the region.
"China seeks hegemony in East Asia," he said.
He added, "China is clearly militarizing the South China Sea, and you'd have to believe in the flat Earth to think otherwise."