With U.S. and South Korean bombers taking to the skies in a show of force, the allies prepared for a possible military clash Saturday as the North's deadline loomed for Seoul to dismantle loudspeakers broadcasting anti-North Korean propaganda across their border.

In the most serious confrontation between the rivals in years, North Korea has declared that its troops are in a "quasi-state of war" and readying for battle if Seoul ignores its Saturday deadline.

It was not clear if North Korea meant to attack immediately, but South Korea has vowed to continue the broadcasts, which it recently restarted after an 11 year stoppage after accusing Pyongyang of planting land mines that maimed two South Korean soldiers earlier this month.

In the South Korean border towns of Yeoncheon, Paju and on Ganghwa Island, officials said they told hundreds of residents to move to shelters ahead of the Saturday afternoon deadline.

Four U.S. F-16 fighter jets and four F-15k South Korean fighter jets simulated bombings, starting on South Korea's eastern coast and moving toward the U.S. base at Osan, near Seoul, officials said.

Ahead of the 5 p.m. (0800 GMT) Pyongyang time deadline, much of Seoul went about its normal business Saturday. More than 240 South Koreans entered a jointly-run industrial complex in the North Korean border city of Kaesong.

Still, there was worry about the latest conflict between the Koreas, seen as the most serious in years. South Korea's military on Thursday fired dozens of artillery rounds across the border in response to what Seoul said were North Korean artillery strikes meant to back up a threat to attack the loudspeakers.

U.S.-based experts on North Korea said the land mine blast and this week's shelling were the most serious security incidents at the border since Kim Jong Un came to power after the 2011 death of his father, Kim Jong Il. The country was founded by Kim Jong Un's grandfather, Kim Il Sung.

"If Kim Jong Il or Kim Il Sung was in charge, I would say that leadership in North Korea would recognize that South Korea has responded in kind to an attack and it's time to stand down. But I'm not sure Kim Jong Un understands the rules of the game established by his father and grandfather on how to ratchet up tensions and then ratchet them down. I'm not sure if he knows how to de-escalate," said Evans Revere, a former senior State Department official on East Asia.

The North denies responsibility for the land mine attack and says it didn't fire across the border, a claim Seoul says is nonsense.

The standoff comes during annual military exercises between the U.S. and South Korea North Korea calls the drills a preparation for invasion, although the U.S. and South Korea insist they are defensive in nature.

Hundreds of residents in South Korean border towns had evacuated to shelters during the conflict on Thursday before returning home on Friday afternoon. Fishermen on Saturday were banned for the second straight day from entering waters near five South Korean islands near the disputed western sea border with North Korea, according to marine police officials in Incheon.

In a propaganda statement carried by North Korea's official Korean Central News Agency, Pyongyang's Foreign Ministry said that North Korean soldiers and people are "poised not to just counter-act or make any retaliation but not to rule out an all-out war to protect the social system, their own choice, at the risk of their lives."

The North's threats are similar to its other warlike rhetoric in recent years. Still, the North's apparent willingness to test Seoul with military strikes and its recent warning of further action raise worries because South Korea has vowed to hit back with overwhelming strength should North Korea attack again.

South Korea's Yonhap news agency, citing an unidentified government source, reported Friday that South Korean and U.S. surveillance assets detected the movement of vehicles carrying short-range Scud and medium-range Rodong missiles in a possible preparation for launches. South Korea's Defense Ministry said it could not confirm the report.

North Korea said the South Korean shells fired Thursday landed near four military posts but caused no injuries. No one was reported injured in the South, either, though hundreds were evacuated from front-line towns. Pyongyang says it did not fire anything at the South, a claim Seoul dismissed as nonsense.

Authoritarian North Korea, which has also restarted its own propaganda broadcasts, is extremely sensitive to any criticism of its government, run by leader Kim Jong Un, whose family has ruled since the North was founded in 1948. The loudspeaker broadcasts are taken seriously in Pyongyang because the government does not want its soldiers and residents to hear outsiders criticize human rights abuses and economic mismanagement that condemns many to abject poverty, South Korean analysts say.

South Korea's military warned Friday that North Korea must refrain from engaging in "rash acts" or face strong punishment, according to South Korea's Defense Ministry. South Korea raised its military readiness to its highest level.

Escalation is a risk in any military exchange between the Koreas because after two attacks blamed on Pyongyang killed 50 South Koreans in 2010, South Korea's military warned that any future North Korean attack could trigger strikes by South Korea that are three times as large.

The Koreas' mine-strewn Demilitarized Zone 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. About 28,500 U.S. soldiers are deployed in South Korea to deter potential aggression from North Korea.

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Kim reported from Seoul, South Korea. Associated Press writers Foster Klug and Hyung-jin Kim in Seoul and Matthew Pennington in Washington contributed to this report.