North Korea stepped up its war rhetoric Thursday, saying its troops are "fully ready" for war with South Korea, just hours before a visit to Seoul by U.S. Secretary of State Hillary Rodham Clinton.

The North's military accused South Korean President Lee Myung-bak of using "nonexistent" nuclear and missile threats as a pretext for an invasion and warned it was prepared for an "all-out confrontation."

The strident statement carried on state-run media comes amid reports that the North is preparing to test-fire a long-range missile and as Clinton heads to Seoul for talks Friday that are expected to focus on North Korea.

Meanwhile, Clinton said that the North's leadership situation is uncertain and the United States is worried the Stalinist country may soon face a succession crisis to replace dictator Kim Jong Il.

Click here for photos.

Speaking to reporters during a flight to South Korea from Indonesia, Clinton said there is "an increasing amount of pressure because if there is a succession, even if it's a peaceful succession, that creates more uncertainty and it may also encourage behaviors that are even more provocative as a way to consolidate power within the society."

Her comments were a rare public acknowledgment from a senior U.S. official that the secretive nation may be preparing for a leadership change following reports that Kim suffered a stroke last year.

Analysts say North Korea is using the threats and missile test preparations to win President Barack Obama's attention at a time when nuclear negotiations with the U.S., South Korea and three other nations stand at a deadlock and tensions with the South are at their highest level in a decade.

North Korea, however, said Monday it "has no need to draw anyone's attention" and has defended its right to use missiles as part of its space program.

Relations between the two Koreas have been tense since Lee took office a year ago taking a harder line toward the North than his liberal predecessors.

The North's military, in a statement carried by the official Korean Central News Agency, called the Lee administration a "group of traitors" and warned it "should never forget that the (North) Korean People's Army is fully ready for an all-out confrontation."

Radio Pyongyang said armed skirmishes could break out at any moment near the Koreas' disputed sea border, according to South Korea's Yonhap news agency, which monitors the broadcasts from Seoul. KCNA later warned that a "physical clash" was just a matter of time.

Dozens rallied outside the U.S. Embassy on Thursday to condemn the North for ratcheting up tensions ahead of Clinton's visit. Anti-Pyongyang protesters burned North Korean flags and photos of leader Kim Jong Il.

"We hope her visit will be a strong message against North Korea's military aggression," said organizer Park Chan-sung.

Clinton's visit comes amid reports that North Korea has moved a Taepodong-2 missile — believed capable of reaching Alaska — to a launch site on its northeast coast.

North Korea says it bears the right to "space development" — a term the regime used in 1998 before conducting a ballistic missile test Pyongyang claims was meant to put a satellite into orbit. The North, which claims to possess atomic bombs, carried out a nuclear test blast in 2006.

Foreign Minister Yu Myung-hwan warned that any such launch would invite U.N. sanctions because it would violate a 2006 resolution banning North Korea from pursuing missile or nuclear programs.

A missile launch "would be very unhelpful in moving our relationship forward," Clinton said Tuesday in Japan.

Clinton has said the Obama administration backs the six-party negotiations to rid North Korea of its nuclear program. The disarmament process, which began in 2007, has been stalled for months.

In a promising sign, representatives from all six nations involved in the nuclear talks were to meet Thursday and Friday in Moscow to discuss promoting regional security. The meeting will be North Korea's first official with the parties since Obama's inauguration.

The halt in the disarmament process last year came amid mounting questions about North Korean leader Kim Jong Il's health. Analysts say Kim, believed to have suffered a stroke in August, wants to extract concessions from the Obama administration before handing over control of the regime to one of his sons.

Kim's youngest son, Kim Jong Un, has registered his candidacy for the March 8 parliamentary elections, Yonhap reported Thursday, citing unnamed sources in Beijing. Yonhap called the move a sign the son has been named Kim's successor.

Yonhap said last month that the son, believed to be in his 20s, was named Kim's heir apparent but the report could not be verified.