Seoul, South Korea (CNN) -- North Korea has reacted angrily to the use of its flag during live-fire drills by South Korea and the United States, calling it "a grave provocative act."
The comments from Pyongyang on Sunday came after the allies held military drills last week less than 50 kilometers (30 miles) away from the North Korean border, involving more than 2,000 military personnel.
An unidentified North Korean foreign ministry spokesman accused South Korea and the United States of firing "live bullets and shells" at the flag, according to a report by the state-run Korean Central News Agency.
The act was "the most vivid expression of their hostile policy," the spokesman said.
The North Korean flag was put on an elevated hill but was not directly used as a target during the exercises, an official for the South Korean Defense Ministry said, declining to be identified.
"It was used only as a symbol of North Korean territory and the drill was a defensive one," he added.
More than 230 military weapons were used in the U.S.-South Korean exercises on Tuesday and Friday last week, including newly upgraded attack helicopters and artillery.
The use of the flag is clearly a provocation for North Korean officials, who will take it as an insult to their identity and dignity, said Choi Jong-kun, a professor at Yonsei University.
"They are not likely to respond physically, but they will definitely capitalize this opportunity for the future," he added.
Pyongyang nonetheless used the military drills as an opportunity to remind Seoul and Washington of its nuclear weapons program.
The North "will further bolster up its nuclear deterrent for self-defence as long as the U.S., the world's biggest nuclear weapons state, persists in its hostile policy," KCNA cited the Foreign Ministry spokesman as saying.
South Korea said last month that satellite images suggested that North Korea was ready to carry out a fresh nuclear test but that the reclusive state was awaiting a "political decision" on whether or not to go ahead.
Many analysts assume an atomic test by North Korea is just a matter of time following the failure of a controversial rocket launch in April. Two previous rocket launches in 2006 and 2009 were followed weeks or months later by nuclear tests.
The two Koreas are still technically at war since the 1950-53 conflict that left the Korean Peninsula divided along a heavily militarized border. The United States has tens of thousands of troops stationed in South Korea.
The live-fire drills were part of a series of military exercises last week, including a trilateral naval drill between the United States, South Korea and Japan.