North Korea is increasingly likely to launch a "surprise provocation" against South Korea following a series of strongly worded threats, Seoul's defence minister said Monday.

"The possibility of a surprise provocation with various means and methods is steadily increasing while (the North is) pressuring us with rhetorical threats," Kim Kwan-Jin told a parliamentary session.

"There have been continuing activities in North Korea to maintain its capability to conduct nuclear tests and launch missiles," Yonhap news agency quoted the minister as saying.

Kim also told legislators the North may have completed the development of smaller nuclear warheads deliverable by missiles or plane.

"It's been a long time (since North Korea's first nuclear test in 2006), so we judge that by this time (it) could have succeeded in making smaller or miniaturised versions of its nuclear weapons," the minister said, adding that his comments were based on suspicions and not firm evidence.

The South is preparing to deploy attack helicopters on the largest of five "frontline" islands near the disputed Yellow Sea border with the North, Yonhap quoted a defence ministry report as saying.

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.

It announced it would have no further dealings with the administration and poured scorn on what it said were secret approaches by Seoul for summit talks.

Its military has also threatened retaliation unless Seoul punishes troops who used pictures of Pyongyang's ruling dynasty as rifle-range targets. The practice has been halted.

Cross-border ties worsened after South Korean President Lee Myung-Bak came to power in February 2008 and linked major aid to his neighbour's nuclear disarmament.

Relations turned icy after the South accused the North of sinking one of its warships in March 2010 with the loss of 46 lives.

The North denies involvement in the sinking. But it shelled a South Korean border island last November, killing two marines and two civilians.

The North says its shelling was provoked by the South's firing drill and refuses to apologise for either incident -- a precondition set by Seoul before any serious dialogue.

Tense cross-border ties are complicating efforts to restart nuclear disarmament talks grouping the two Koreas, China, the United States, Japan and Russia.

The North abandoned them in April 2009, a month before its second atomic test. China and other members suggest inter-Korean talks as a first step to reviving the wider dialogue.

The South will no longer condone the North's "tactics of brinkmanship" and the six-party talks should resume only if they can make progress, Seoul's Foreign Minister Kim Sung-Hwan told a security forum.

"North Korea must demonstrate its sincerity toward denuclearisation through concrete actions and thereby restore the trust of the international community prior to the resumption of the six-party talks," he said in a speech.

The foreign minister also urged the international community to define the North's uranium enrichment programme disclosed last November as illegal under UN resolutions.

Pyongyang says the programme is peaceful but experts say it could offer the regime a second way to make nuclear weapons.