South Korean troops fired at a passenger jet flying from China with 119 people on board after mistaking it for a North Korean aircraft, amid increasingly fraught relations on the divided peninsula.

Two soldiers at a guard post on Gyodong island, just 1.7 kilometres (one mile) south of the North Korean coast, fired their K-2 rifles on Friday towards the plane, descending as it approached Seoul's Incheon International Airport.

Ties between the two Koreas are at their lowest ebb in more than a decade after Pyongyang announced late last month it was breaking all contacts with the South's conservative government.

The South Korean Asiana aircraft was flying southeast over Jumun island, 12 kilometres south of Gyodong, towards Incheon, when the soldiers fired a total of 99 rounds including two blanks, Yonhap said.

"The firing continued about 10 minutes but the plane was too far off the rifle's range and it did not receive any damage," Yonhap news agency quoted a Marine Corps official as saying.

"When the plane appeared over Jumun island, soldiers mistook it as a North Korean military aircraft and fired."

A Marine Corps spokesman confirmed the incident to AFP but declined to give further details.

An aviation controller told AFP that the Asiana flight from the southwest Chinese city of Chengdu, carrying 119 people including crew members and passengers, was following a normal route.

"It was flying normally. It did not deviate from its normal route," the controller said.

An Asiana spokesman said there was no damage as the plane was too far away from the military guard post, adding that the military had inquired whether the plane had received any damage.

South Korean soldiers had been alerted to possible provocative acts by North Korea amid simmering cross-border tensions.

After a few months of relative calm, the North since late May has been using harsher rhetoric against the South's conservative government -- describing it as a US puppet bent on fuelling confrontation.

The soldiers gave a report about the incident to their seniors within five to 10 minutes after the incident, Yonhap said.

Defence Minister Kim Kwan-Jin has told frontline troops that if North Koreans attack, they are to strike back immediately without waiting for orders from top commanders about how to respond.

"Don't ask your commanders whether to fire back or not. Take actions first and then report afterwards," Kim was quoted as saying when he visited the western frontline in March.

South Korean authorities on Friday rejected North Korea's demand to send back nine refugees from the communist state.

The three men, two women and four children crossed the Yellow Sea border last Saturday in two small boats, with the North's Red Cross warning Thursday that relations could worsen unless the South immediately sends them back.

But Seoul's policy is to accept all North Koreans who wish to stay in the South, while repatriating those who stray across the sea border by accident.

The arrival in February of a boatload of North Koreans sparked weeks of acrimony. That boat drifted across the Yellow Sea border in thick fog, possibly accidentally.

Seoul returned 27 of the 31 people on board but refused to hand over the other four, saying they had freely chosen to stay in the South.

Pyongyang complained bitterly that the four had been pressured to stay and publicised appeals from their relatives for them to come home.

And the North's military threatened an attack in protest at the use by some South Korean troops of photos of Pyongyang's ruling family as rifle-range targets. The practice has been stopped but the North is demanding an apology.

Media reports on Friday said the South has deployed missiles capable of hitting the North Korean capital Pyongyang near the tense border.

The deployment of the surface-to-surface missiles was in response to a recent rise in tensions, Yonhap news agency and Dong-A Ilbo newspaper reported.