SEOUL?Pyongyang accused Seoul on Monday of plotting to topple its regime, souring relations again as a top Chinese official visited North Korea in a new drive to bring it back to nuclear disarmament talks.

In another blow for inter-Korean ties the two sides failed to agree on restarting a tourism project.

Wang Jiarui, head of the Chinese Communist Party's international department, met senior communist party official Choe Thae-Bok to reaffirm the two countries' friendship and exchange views "on other issues of common concern," Beijing's Xinhua news agency said.

The meeting came one day before Lynn Pascoe, top political adviser to UN chief Ban Ki-moon, was due in Pyongyang for a four-day visit.

South Korean media forecast that Wang, who has met leader Kim Jong-Il several times in the past, would do so again before he departs Tuesday.

China hosts the six-party nuclear talks which its ally North Korea quit last April, a month before staging a second nuclear test.

As conditions for returning to the nuclear forum, the North wants a US agreement to hold formal peace talks and a lifting of UN sanctions.

In an apparent conciliatory gesture to Washington, Pyongyang on Saturday freed a US missionary who had crossed the border last December 25 on a lone campaign to publicize rights abuses.

The message to Seoul was less conciliatory.

A statement from two security ministries said Pyongyang has a secret strike force to counter what it called Seoul's plots.

"We have world-level ultra-modern striking force and means for protecting security which have neither yet been mentioned nor opened to the public in total," the statement on official media said without elaborating.

The North criticized efforts by the South's military to defend the disputed Yellow Sea border -- where the North fired artillery salvoes late last month -- and its "reckless" operations to destabilize the North.

It complained about "the daily escalating" scattering of propaganda leaflets by balloon, which were now penetrating deep into the country from border areas.

Despite the tough talk, the North has been pushing to revive business projects with the South since it was hit by tougher sanctions for its missile launches and nuclear test last year.

The two sides held talks Monday about a possible resumption of tours which previously earned the cash-strapped state tens of millions of dollars a year.

Seoul suspended the trips after soldiers in July 2008 shot dead a Seoul housewife who strayed into an off-limits military zone at the Mount Kumgang resort in the North.

South Korea's unification ministry demands safety guarantees before it restarts them.

But spokesman Chun Hae-Sung said Monday's meeting in the North Korean border town of Kaesong ended "without any significant agreement" after Pyongyang rejected Seoul's terms.

Chun said South Korea demanded that its officials conduct an on-site probe into the shooting but "the North Korean side said it has already conducted sufficient investigations into the incident".

The South also demanded the North guarantee the safety of future tourists. The North said such steps were taken when Kim Jong-Il met last August with a South Korean businesswoman whose company runs the tours.

About 1.9 million visitors, mainly South Koreans, have visited the Seoul-funded Kumgang resort since it opened in 1998. Over a decade the tours earned the North a total of 487 million dollars.