U.S. Defense Secretary Leon Panetta on Monday lashed out at North Korea for "reckless and provocative" acts and criticized China for a secretive expansion of its military power.

Panetta, who arrived at this U.S. air base on the second leg of a weeklong Asia tour, spoke out about North Korea and China in an opinion piece published Monday by Japan's Yomiuri newspaper before his arrival.

North Korean and U.S. negotiators met on Monday for direct talks aimed at reviving long-stalled nuclear disarmament negotiations.

The U.S. team led by outgoing special representative Stephen Bosworth, as well as his replacement Glyn Davies arrived shortly before 8.30am (06:30 GMT) at the U.S. embassy where the meeting was to begin at 10.00am.

The United States said Wednesday it will hold rare direct talks with North Korea next week on ending the authoritarian state's nuclear program and announced it was replacing its chief envoy.

The State Department said that U.S. and North Korean officials will meet Monday and Tuesday in Geneva but insisted that the talks were "exploratory" and that Pyongyang needed to offer proof that it was serious about dialogue.

North Korea's military on Saturday threatened retaliation against "provocative" acts from the South including the scattering of anti-Pyongyang propaganda leaflets across the border.

The warning came as activists in South Korea opposed to the secretive regime plan to float balloons carrying the leaflets across the border in time for the 66th anniversary of the North's ruling communist party inauguration on Monday.

North Korea must shut down its newest nuclear program before long-stalled six-nation disarmament talks can resume, Seoul's top presidential security aide said Friday.

Chun Yung-Woo, senior secretary for foreign affairs and national security, said other parties in the talks would not reward the North for abandoning its "illegal" nuclear activities.

South Korea's military on Thursday fired thousands of shells during a live-fire artillery drill near the tense Yellow Sea border with North Korea, the defense ministry said.

Troops fired more than 5,000 rounds from K-9 self-propelled guns, Vulcan cannon and other artillery during the two-hour morning exercise, a ministry spokesman told Agence France Presse.

A senior North Korean official is due to visit the United States amid signs of an easing of relations between the two countries, a report said Monday.

Ri Jong-Hyok, vice chairman of the Korea Asia-Pacific Peace Committee, has been allowed to attend a forum at the University of Georgia on October 17-20, Yonhap news agency said.

The United States and North Korea are likely to hold a second round of talks this month to try to revive international nuclear disarmament negotiations, a South Korean report said Sunday.

The meeting may come after a summit between U.S. President Barack Obama and his South Korean counterpart Lee Myung-Bak in Washington on October 13, an unnamed senior government official in Seoul told Yonhap news agency.

South Korea's ruling-party chief Friday began a rare visit to North Korea by a senior Seoul politician, saying he hopes to break a long stalemate in cross-border relations.

Hong Joon-Pyo, chairman of the conservative Grand National Party (GNP), crossed the closely guarded border to pay a one-day visit to a jointly run industrial estate at Kaesong in the North.

Chinese Premier Wen Jiabao welcomed North Korean Prime Minister Choe Yong-Rim to Beijing Monday to start talks following on from a recent visit by leader Kim Jong-Il to China, his fourth in 16 months.

North Korea's economic dependence on China, its sole major ally, has grown increasingly important since South Korea froze most contacts with its neighbor.