South Korea ramped up border security Tuesday as military tensions flared following landmine blasts blamed on North Korea, and the presidential office in Seoul demanded a formal apology.

South Korea says North Korean soldiers sneaked across the border and laid the mines, three of which were tripped by members of a South Korean border patrol on Tuesday last week.

One soldier underwent a double leg amputation while another had one leg removed.

The South responded by resuming border propaganda operations after a break of more than a decade, switching on batteries of powerful loudspeakers to blare out messages denouncing border provocations.

North Korea is extremely sensitive to such campaigns. The last time the South threatened to turn the loudspeakers back on -- in 2010 -- the North vowed to shell the units involved.

"We are strengthening defence postures (along the border) against another potential provocation by the North," Seoul's defence ministry spokesman Kim Min-Seok said Tuesday.

The army will "respond immediately" if the North opens fire at the loudspeakers, Kim said, adding that border area residents had been advised to exercise extreme caution and farmers to leave their fields.

Until now, there has been no unusual North Korean activity observed along the border.

The mine blasts came with cross-border tensions already running high ahead of the launch next week of a major South Korea-US joint military exercise condemned by Pyongyang.

In Seoul, the presidential Blue House demanded an apology for what it called a "clear breach" of the armistice agreement that ended the 1950-53 Korean War.

"We sternly urge North Korea to apologise for this provocation and punish those responsible," Blue House spokesman Min Kyung-Wook told reporters.

Because the 1953 armistice was never replaced with a peace treaty, the two Koreas remain technically at war.

British Foreign Secretary (foreign minister) Philip Hammond, who is on a two-day trip to South Korea, condemned the mine blasts as a violation of the armistice and a threat to regional peace.

"We've condemned this unprovoked attack. And North Koreans must be held to account for the breach of the armistice," he told reporters during a trip to a memorial honouring British servicemen killed during the inter-Korean conflict.

The North's actions "threaten stability in this region", Yonhap news agency quoted Hammond as saying.

The defence ministry declined to comment on how many units were involved in the propaganda broadcasts, which resumed late Monday afternoon.

Media reports suggested loudspeakers had been switched on at up to 11 locations along the border.

A ministry official said the messages being boomed across the border ranged from snippets of world news and the weather forecast to the superiority of democracy.

He said noise from the speakers could be heard 10-20 kilometres (6-12 miles) away depending on the time of day.

Both Koreas discontinued the high-decibel propaganda exchanges in 2004 during a period of rapprochement.

But South Korean civil activists have continued -- much to Pyongyang's fury -- to send anti-North leaflets over the border using helium balloons.