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 . North Korea slams joint drill among U.S. , Japan and South Korea . `` 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 . Clinton says young North Korean leader 'has a choice '
The United States and South Korea hold a joint military exercises . A North Korean flag is used in the live-fire drills . Pyongyang says that constitutes a grave provocation . South Korea says the flag was to mark territory and was not used as a target .