South Korea warned that North Korea was likely to launch "provocations" if Seoul did not meet a Saturday deadline to cease propaganda broadcasts as tensions heightened on the divided peninsula Friday.

South Korean Defense Minister Han Min-koo issued the warning at a press conference as a South Korean media outlet reported that Pyongyang appeared to be preparing to test-fire short- and mid-range ballistic missiles. A senior military officer in North Korea held a rare "emergency situation briefing" for diplomats and military attaches in Pyongyang.

The report by Yonhap News Agency cited a South Korean government source who said that North Korea seemed to be "weighing the timing of the firing under its strategic intention to increase military tension on the Korean Peninsula to the highest level." The source also said that the apparent preparations for the test had been detected by South Korea's joint radar system, which it shares with the United States.

The North has given Seoul a deadline of 5 p.m. Saturday evening (4 a.m. EDT) to remove border loudspeakers that—after an 11 year lull-- have started broadcasting anti-Pyongyang propaganda. Failure, Pyongyang says, will result in further military action. Seoul has vowed to continue the broadcasts.

Earlier Friday, North Korean leader Kim Jong-un declared his country to be in a "quasi-state of war" and fully ready for any military operations starting Friday evening, according to a report by Pyongyang's official Korean Central News Agency.

In response, 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.

The North's media report said that "military commanders were urgently dispatched for operations to attack South Korean psychological warfare facilities if the South doesn't stop operating them." South Korea's vice defense minister said Friday this likely meant the North would fire on the 11 sites where South Korea had set up loudspeakers to broadcast propaganda.

The loudspeaker broadcasts began after South Korea accused the North of planting land mines that maimed two South Korean soldiers earlier this month. One of the injured soldiers lost both legs and the other one leg. North Korea denies the South's accusation and demanded video proof.

The North's declaration Friday is similar to its other warlike rhetoric in recent years, including repeated threats to reduce Seoul to a "sea of fire," and the huge numbers of soldiers and military equipment already stationed along the border mean the area is always essentially in a "quasi-state of war." 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.

The North's capital of Pyongyang was mostly business as usual Friday morning, although propaganda vans with loudspeakers broadcast the state media line that the country was in a "quasi-state of war" to people in the streets.

North Korea on Thursday afternoon first fired a single round believed to be from an anti-aircraft gun, which landed near a South Korean border town, Seoul said. About 20 minutes later, three North Korean artillery shells fell on the southern side of the Demilitarized Zone dividing the two Koreas. South Korea responded with dozens of 155-millimeter artillery rounds, according to South Korean defense officials.

North Korea said the South Korean shells landed near four military posts but caused no injuries. No one was reported injured in the South, either, though hundreds were evacuated from frontline towns.

On Friday, about 60 residents in the South Korean town near where the shell fell, Yeoncheon, were still in underground bunkers, Yeoncheon officials said. Yonhap reported that a total of about 2,000 residents along the border were evacuated Thursday.

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.

Many in Seoul are accustomed to ignoring or discounting North Korea's repeated threats, but the latest have caused worry because of Pyongyang's warning of strikes if the South doesn't tear down its loudspeakers by Saturday evening. Observers say the North may need some save-facing measure to back down.

This is what happened in December 2010, when North Korea backed off an earlier warning of catastrophic retaliation after South Korea defiantly went ahead with live-fire drills near the country's disputed western sea boundary. A month earlier, when South Korea staged similar drills, the North reacted with an artillery bombardment that killed four people on a South Korean border island. North Korea said it didn't respond to the second drill because South Korea conducted it in a less provocative way, though the South said both drills were the same.

The rivals are currently at odds also over annual U.S.-South Korean military drills that North Korea calls an invasion rehearsal. Seoul and Washington say the drills are defensive in nature.

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. About 28,500 U.S. soldiers are deployed in South Korea to deter potential aggression from North Korea.

The Associated Press contributed to this report.