North Korea has branded the use of its national flag in U.S.-South Korean military drills a "grave provocative act" and an insult, Belfast Telegraph said.

An unidentified Foreign Ministry spokesman said in a statement Pyongyang will continue boosting its nuclear weapons programme because of U.S. 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 U.S. 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.