North Korea on Friday urged a sceptical South Korea to respond to a recent series of trust-building gestures and again called on Seoul to cancel upcoming military drills with the United States.

The latest apparent olive branch came in the form of an "open letter" sent to the South Korean authorities by the North's top military body on the direct orders of leader Kim Jong-Un in an effort to promote "reconciliation and unity".

Carried by the North's official KCNA news agency, the letter followed up on a series of confidence-building proposals that South Korea has already dismissed as a "deceptive" propaganda exercise.

"What is important for paving a wide avenue for mending North-South relations is to make a bold decision to stop all hostile military acts, the biggest hurdle stoking distrust and confrontation," the letter from the National Defence Commission (NDC) said.

A week earlier, the NDC had sent several proposals, urging South Korea to cancel the joint exercises with the United States and offering a mutual moratorium on mud-slinging by the two rivals.

Seoul not only dismissed the overtures, but warned that Pyongyang may well be laying the ground for a military confrontation.

"Regretfully, the South Korean authorities still remain unchanged in their improper attitude and negative stand," the NDC letter said.

The South "should not thoughtlessly doubt, misinterpret and rashly reject our sincere, important proposal," it added.

S. Korea warns of 'hidden motive'

South Korea's Unification Ministry said it would respond to the letter later Friday, while Defence Ministry spokesman Wi Yong-Seop warned of the "enemy's hidden motive."

Temperatures on the Korean peninsula traditionally rise ahead of the annual South Korean-US drills, which Pyongyang routinely condemns as a rehearsal for invasion.

Last year they coincided with an unusually sharp and protracted surge in tensions, which saw the North threatening pre-emptive nuclear strikes, and nuclear-capable US stealth bombers flying practice runs on the peninsula.

In its letter, the NDC stressed that its opposition lay solely in the participation of US forces in the exercises.

North Korea "did not urge the South Korean authorities to stop ordinary military drills," it said.

"It urged them to halt drills for a war of aggression to be staged against their compatriots in collusion with outside forces."

The NDC said it had also taken the "unilateral" step of halting all cross-border "slandering", despite the South's dismissive response to its proposal a week ago.

The South's Unification Ministry had scoffed at the idea, arguing that the only "slander" was propagated by Pyongyang's propaganda machine.

Scepticism over charm offensive

Many analysts have voiced scepticism over the North's recent charm offensive, noting its past proclivity for offering conciliatory gestures prior to an act of provocation.

The trust-building gestures are bogus, the analysts say, because they are founded on the un-realisable demand that President Park Geun-Hye's conservative administration call off the joint military drills set to begin late February.

"The North Koreans know full well that demand would be completely unacceptable, even to a more left-leaning, pro-engagement administration in Seoul," said Andrei Lankov, a North Korea scholar at Seoul's Kookmin University.

Kim Yong-Hyun, a North Korean expert at Seoul's Dongguk University, said the North was pre-emptively seeking to shift the blame for any future confrontation by making South Korea appear intransigent.

"It wants the world to believe that the South is avoiding dialogue while the North is seeking to improve relations," Kim said.

President Park says she is willing to hold a summit with Kim Jong-Un under the right conditions, but insists that a substantive dialogue can only begin when North Korea shows a tangible commitment to abandoning its nuclear weapons programme.

The NDC letter underlined the North's desire for denuclearisation, but argued that the real obstacle was South Korea.

"Before finding fault with the precious nuclear force for self-defence to which (North Korea) has access, they should make a bold decision to stop their dangerous acts of introducing outsiders' nukes," it said, referring again to the military exercises.

Under its defence agreement with Washington, South Korea is protected by the US nuclear umbrella and the United States would assume overall operational command of joint US and South Korean forces if a full-scale conflict with the North broke out.

President Park has ordered the South Korean military to maintain an "airtight" defence posture in expectation of a possible act of aggression from the North.

The JoongAng Daily newspaper on Friday cited an unidentified South Korean government official as saying that recent North Korean military exercises had included a simulated attack on the South's main international airport at Incheon.