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Ahn Young-joon, Associated Press
A South Korean resident watches his mobile phone to read news as people gather at a shelter in the South Korean town of Yeoncheon where the shell fell Thursday, Aug. 20, 2015. South Korea's military fired dozens of shells Thursday at rival North Korea after the North lobbed a single artillery round at the border town, the South's Defense ministry said.

PYONGYANG, North Korea — South Korea fired dozens of shells Thursday at rival North Korea after the North lobbed several rounds across the world's most heavily armed border and threatened to take further action unless Seoul ends its loudspeaker broadcasts. The North denied it fired any shots and warned of retaliation for what it called a serious provocation.

Officials in Seoul said the North fired across the Demilitarized Zone to back up an earlier threat to attack South Korean border loudspeakers that, after a lull of 11 years, have started broadcasting anti-Pyongyang propaganda. But the Supreme Headquarters of the Korean People's Army issued a statement later Thursday denying it had launched any shots at the South.

"Using the pretext that our forces fired one shell to the south, which is not true, it made reckless moves by firing 36 shells at our military posts," said the statement, published in Korean by the North's state media. It said the shells landed near four military posts, but caused no injuries.

"This reckless shelling incident is a serious military provocation to our sacred territory and military posts which is intolerable," it said.

The broadcasts began after South Korea accused the North of planting land mines that maimed two South Korean soldiers earlier this month.

North Korea first fired a single round believed to be from an anti- aircraft gun, which landed at a South Korean border town on Thursday afternoon. About 20 minutes later, several more artillery shells fell on the southern side of the Demilitarized Zone dividing the two Koreas.

South Korea responded with dozens of 155-milimeter artillery rounds, according to South Korean defense officials.

There were no reports of casualties, and North Korea didn't respond militarily to South Korea's artillery barrage. But the North's army later warned in a message that it will take further military action within 48 hours if South Korea doesn't pull down the loudspeakers, according to South Korea's Defense Ministry.

South Korea raised its military readiness to its highest level.

Joint Chiefs of Staff spokesman Jeon Ha-kyu told a televised news conference that South Korea is ready to repel any additional provocation. Defense officials said South Korea will continue loudspeaker broadcasts despite the North Korean threats.

Authoritarian North Korea, which has also restarted its own propaganda broadcasts, is extremely sensitive to any criticism of the government run by leader Kim Jong Un, whose family has ruled since the North was founded in 1948. Pyongyang worries that the broadcasts could weaken Kim's grip on absolute power, analysts say.

The artillery exchange also comes during another point of tensions between the Koreas: annual U.S.-South Korean military drills that North Korea calls an invasion rehearsal. Seoul and Washington say the drills are defensive in nature.

South Korean President Park Geun-hye convened an emergency National Security Council meeting and ordered South Korea's military to "resolutely" deal with any provocation by North Korea. North Korea. North Korea, meanwhile, convened an enlarged emergency meeting of the central military commission of its ruling party.

In Washington, State Department spokesman John Kirby said the U.S. was concerned by the firing of a projectile into South Korea from the North and was closely monitoring the situation.

"These kinds of provocative actions only heighten tensions and we call on Pyongyang to refrain from actions and rhetoric that threaten regional peace and security," Kirby said, reiterating the U.S. commitment to its security alliance with South Korea.

About 80 residents in the South Korean town where the shell fell, Yeoncheon, were evacuated to underground bunkers, and authorities urged other residents to evacuate, a Yeoncheon official said, requesting anonymity because he wasn't authorized to speak to the media.

In the nearby border city of Paju, residents were asked to stay home. On Ganghwa Island, residents in villages near a site where South Korea operates one of its loudspeakers were also evacuated, according to island officials.

South Korea's Yonhap news agency reported that a total of about 2,000 residents along the border were evacuated.

Pyongyang says that Seoul fabricated its evidence on the land mines and demanded video proof.

While the Koreas regularly exchange hostile rhetoric, it is also not unusual for fighting to occasionally erupt. Last October, North Korean troops opened fire at areas in Yeoncheon, after South Korean activists launched balloons there that carried propaganda leaflets across the border. South Korea returned fire, but no casualties were reported. Later in October, border guards from the two Koreas again exchanged gunfire along the border, without any casualties.

Before that, the Koreas tangled in a deadly artillery exchange in 2010, when North Korean artillery strikes on a South Korean border island killed four South Koreans. Earlier in 2010, an alleged North Korean torpedo attack killed 46 South Korean sailors.

North Korea's army said recently in a statement that the South Korean propaganda broadcasts were a declaration of war and that if they were not immediately stopped "an all-out military action of justice" would ensue.

South Korea has said the two soldiers wounded from the mine explosions were on a routine patrol in the southern part of the DMZ that separates the two Koreas. One soldier lost both legs and the other one leg.

The Koreas' mine-strewn DMZ is a legacy of the 1950-53 Korean War, which ended with an armistice, not a peace treaty, leaving the Korean Peninsula still technically in a state of war.

AP writers Hyung-jin Kim in Seoul and Matthew Pennington in Washington contributed to this story.