North Korea said Wednesday that it is making rapid progress on work to enrich uranium and build a light-water nuclear power plant, increasing worries that the country is developing another way to make atomic weapons.

An unidentified spokesman at Pyongyang's Foreign Ministry said in a statement that the construction of an experimental light-water reactor and low enriched uranium are "progressing apace."

North Korea threatened Thursday to turn South Korea's presidential palace into a "sea of fire" in response to any provocation, a day after Seoul's military held a big exercise near the border.

The land, sea and air drill was staged to mark the first anniversary of a deadly North Korean attack on the South's border island of Yeonpyeong, which sparked outrage among South Koreans and prompted international alarm.

South Korea's nuclear envoy Lim Sung-Nam will meet a top U.S. official in Vienna next week for talks on ways to revive stalled negotiations on North Korea's nuclear disarmament, the foreign ministry said Thursday.

The meeting with Glyn Davies, new U.S. special representative for North Korea policy, will be held during Lim's two-day trip to Vienna from Monday, the ministry said.

Iranian cleric Ayatollah Ahmed Khatami warned the chief of the International Atomic Energy Agency on Monday not to become "an instrument without will in the hands of the United States" against Iran.

The hardline cleric hit out at IAEA director general Yukiya Amano in an address during communal prayers in Tehran marking the Muslim Eid al-Adha feast.

The Iranian government has mastered the critical steps needed to build a nuclear weapon after receiving assistance from foreign scientists, The Washington Post reported late Sunday.

Citing unnamed Western diplomats and nuclear experts familiar with new intelligence to be released to the United Nations, the newspaper said a former Soviet weapons scientist had allegedly tutored Iranians on building high-precision detonators of the kind used to trigger a nuclear chain reaction.

A group of 21 North Koreans including women and children have been found adrift off the South's western coast, military authorities said Saturday.

A five-ton wooden boat with the group on board was spotted by sailors on a South Korean Navy vessel in the Yellow Sea on Sunday, the Joint Chiefs of Staff said in a press statement. Earlier reports said they were found Tuesday.

U.N. investigators have identified a previously unknown complex in Syria that bolsters suspicions that the Syrian government worked with A.Q. Khan, the father of Pakistan's atomic bomb, to acquire technology that could be used to make nuclear arms.

The buildings in northwest Syria closely match the design of a uranium enrichment plant provided to Libya when Moammar Gadhafi was trying to build nuclear weapons under Khan's guidance, officials told The Associated Press.

The U.S. and South Korean defense chiefs vowed Friday to raise combat-readiness against North Korean attacks following two deadly incidents last year, saying aggression "is not to be tolerated".

Leon Panetta and his counterpart Kim Kwan-Jin also described the North's uranium enrichment program revealed last November as a "grave threat" that gave it a second path to develop atomic weapons.

A senior U.S. official said Thursday there was some progress made but "no breakthrough" at this week's meeting with North Korea aimed at restarting long-stalled nuclear disarmament talks.

U.S. Assistant Secretary of State Kurt Campbell was speaking after briefing South Korea about the talks in Geneva on Monday and Tuesday.

U.S. Defense Secretary Leon Panetta began a visit to South Korea Wednesday, pledging to keep a "nuclear umbrella" in place over Washington's close ally to deter threats from North Korea.

Panetta, on the last leg of an Asian tour which also took him to Indonesia and Japan, was to meet troops at the huge Yongsan U.S. army base in central Seoul later in the day.