North Korea has branded the use of its national flag in US-South Korean military drills a "grave provocative act" and an insult.
An unidentified Foreign Ministry spokesman said in a statement Pyongyang will continue boosting its nuclear weapons programme because of US hostility.
The statement calls the programme "an all-powerful treasured sword for preventing a war".
The drills on Friday were the allies' biggest since the Korean War.
A huge North Korean flag on a hill disappeared behind flames and smoke as South Korean jets and US helicopters fired rockets.
The UN Security Council condemned a North Korean rocket launch in April that Seoul and Washington called a cover for a test of banned long-range missile technology.
North Korea says it was meant to launch a satellite.