North Korea on Tuesday criticised South Korea's rejection of its latest call for dialogue and insisted it is sincere in trying to mend relations after months of tensions.

Washington and Seoul remained sceptical of the peace overtures, saying they must be backed by action.

The communist state late Monday officially proposed dialogue with Seoul, less than two months after it launched a deadly bombardment of a border island.

The South rejected the offer. It said it was willing to hold government-level talks but the North must admit responsibility for a series of provocations and confirm a commitment to scrapping its nuclear programme.

Pyongyang's government newspaper Minju Josun accused Seoul of dampening hopes for improved relations.

"Those who question our sincerity need to read our statement (offering dialogue) thoroughly. It offered extensive talks and negotiations with no conditions attached," said an editorial carried by the official news agency.

"Whether we have sincerity or not will be confirmed once we sit down face to face," the newspaper said, urging Seoul to "immediately stop libels and provocations".

The November 23 attack on the South's Yeonpyeong island killed four people including civilians, and sparked fears of broader conflict on the peninsula.

But after weeks of high tensions and warnings by the North of nuclear war, the regime changed tack and called in a New Year message for improved relations.

"Now we clearly see the North's repeated pattern of provocations followed by peace overtures," Seoul's Unification Minister Hyun In-Taek told a forum.

"It's regrettable that the North, while offering dialogue, admits no responsibility for the casualties among our people. And we can't see any sincerity in such a stance."

Apart from the island attack, the South accuses the North of torpedoing a warship last March with the loss of 46 lives. It denies the charge.

Hyun said the North "needs to bring down the walls created by itself and show determination and actions on denuclearisation".

Pyongyang has expressed conditional willingness to return to six-nation nuclear disarmament talks that it abandoned in April 2009, a month before staging a second nuclear test.

But it fuelled regional fears in November by disclosing a uranium enrichment plant which could potentially give it a second route to a bomb.

The US State Department said Monday the North has entered "the charm stage" after a year of provocations.

"We rode through last year a provocative stage. We're now in the charm stage. But, you know, the charm stage has to be followed up with a real demonstration that North Korea's prepared for sustained and constructive dialogue," said spokesman Philip Crowley.

"First and foremost, if North Korea makes a public pledge not to attack South Korea or undertake further provocations that threaten South Korea, that would be a significant step to improve the environment," Crowley said.