North Korean leader Kim Jong-un has ordered his "troops to be fully ready for any military operations at any time from 5 p.m. [GMT] Friday," North Korea's official Central News Agency has reported.

The announcement follows an emergency meeting of the country's military after days of heightened tensions and violence between North and South Korea.

South Korea fired dozens of artillery rounds toward North Korea on Thursday after the North launched shells to protest Seoul's anti-Pyongyang propaganda broadcasts along the border.

North Korea did not return fire but later declared a "quasi-state of war in frontline areas" facing the rival South and warned Seoul in a letter that it would take military action if the South did not stop the loudspeaker broadcasts within 48 hours (5 p.m. on Saturday), South Korea's defence ministry said.

In a separate letter, Pyongyang said it was willing to offer an opening to resolve the conflict even though it considers the broadcasts a declaration of war, South Korea's Unification Ministry said.

A South Korean military official said the broadcasts, which began on Aug. 10, would continue.

The Supreme Headquarters of the Korean People's Army issued a statement later Thursday denying it had launched any shells 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.

South Korea said the North fired a 14.5-mm anti-aircraft shell at 3:52 p.m. (6:52 a.m. GT), then fired multiple shells from a 76.2 mm direct fire weapon at 4:15 p.m.

No damage or injuries were reported in the South.

South Korean President Park Geun-hye told top defence officials to "react firmly" to North Korean provocations, a spokesman quoted her as saying.

A South Korean soldier uses a radio on a military vehicle at the South Korean border town of Yeoncheon on Thursday. (Hong Hae-in/Associated Press)

Highest alert

South Korea's military, which said it fired 155-mm artillery rounds in response, raised its alert status to the highest level.

"Our military has stepped up monitoring and is closely watching North Korean military movements," South Korea's defence ministry said.

There was no mention of the firing in isolated North Korea's state media, which does not typically make immediate comment on events.

The first North Korean shell landed in an area about 60 kilometres north of Seoul in the western part of the border zone, the defence ministry said. Nearly 800 South Korean residents living close to the border were ordered to head to shelters, according to officials from Gyeonggi province and the city of Incheon.

South Korean residents gather at a shelter in the South Korean town of Yeoncheon on Thursday. (Ahn Young-joo/Associated Press)

The exchange of fire was the first between the two Koreas since last October, when North Korean soldiers approached the military border and did not retreat after the South fired warning shots, the South Korean Defence Ministry said at the time. The North's soldiers fired back in an exchange of gunfire that lasted about 10 minutes, with no casualties.

Tension between the two Koreas has risen since early this month when landmine explosions in the demilitarized zone [DMZ] of the border wounded two South Korean soldiers. Seoul accused North Korea of planting the mines, which Pyongyang has denied.

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.

U.S.-South Korea alliance

South Korea and the United States are holding annual military drills that the North has long said are a rehearsal for the invasion of their country.

In Washington, State Department spokesman John Kirby said the United States 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.

Onlookers look at a police dog as they take part in an anti-terror drill in Seoul, South Korea. South Korea staged a nationwide joint civil defence drill alongside U.S. forces on Wednesday. (Kim Hong-Ji/Reuters)

North Korea is asking the United Nations Security Council to put the ongoing military exercises by the U.S. and South Korea on the council's agenda as a matter of international peace and security.

North Korea's letter, dated Wednesday from Ambassador Ja Song Nam to the council president, says the Security Council has "unjustifiably ignored" similar requests. It wants an urgent meeting of the council.

Duelling broadcasts

North Korea on Saturday demanded that the South stop the broadcasts or face military action, and on Monday began conducting its own broadcasts.

The two Koreas have remained in a technical state of war since the 1950-53 Korean War ended in a truce, not a peace treaty.

South Korea's won currency weakened in non-deliverable forward trading on the reports of the firing, which came after onshore spot trading had closed. The one-month contract rose as high as 1,192.7 won per dollar from around 1,189.8 earlier.