SEOUL (Reuters) - South Korea and the United States raised the military alert level for the peninsula on Thursday after the communist North warned the truce ending the Korean War was dead and it was ready to attack.

North Korea ramped up tensions this week with a series of provocations rarely seen since the 1950-53 Korean War, including war threats, missile launches and a nuclear test that puts it closer to having an atomic bomb.

The joint command for the 28,500 U.S. troops that support South Korea’s 670,000 soldiers has raised its alert a notch to signify a serious threat from North Korea, the South’s Joint Chiefs of Staff said.

It is the highest threat level since the North’s only other nuclear test in October 2006.

North Korea looks certain to face fresh sanctions for defying a U.N. resolution by exploding a nuclear device for a second time, Western diplomat said, and a vote in the 15-nation Security Council could come as early as next week.

North Korea could be set for further provocations that include additional short-range missile tests off its west coast, the South’s Yonhap news agency on Wednesday night quoted an unnamed government source as saying.

Analysts said the North’s saber-rattling might be partly aimed at firming leader Kim Jong-il’s grip on power and helping him draw up succession plans in Asia’s only communist dynasty after a suspected stroke in August raised questions over his rule.

Weapons experts point out that while North Korea is pushing hard to develop a nuclear arsenal, it does not have an effective way to attack with an atomic warhead or bomb.

Security Council powers have agreed in principle that North Korea must face expanded sanctions, Western diplomats said on Wednesday. Russia confirmed it supported a new resolution, although it said it was too early to discuss penalties.

Possible steps include a ban on importing and exporting all arms and not just heavy weapons, asset freezes and travel bans for North Korean officials, and placing more firms on a U.N. blacklist.

The measures would expand on sanctions approved by the council after Pyongyang’s 2006 nuclear test, penalties that have been widely ignored and left unenforced.

Washington is deeply concerned about North Korea spreading its nuclear technology abroad and the diplomats said cargo inspections were also possible. China, fearing instability in its neighbor , is reluctant to endorse this approach.

U.S. officials have urged China to pressure North Korea to step back from nuclear brinkmanship and return to stalled disarmament talks. But many Chinese analysts say Washington overstates Beijing’s sway over Pyongyang, as well as their government’s willingness to use that influence.

Slideshow ( 22 images )

“Undoubtedly, China also wants a swift and united response, but it probably won’t give the United States all it wants. China has its own worries,” said Shi Yinhong, an expert on regional security at Renmin University in Beijing.

MILITARY ON ALERT

North Korea, which has only become poorer since Kim took over in 1994, has been punished for years by sanctions and is so destitute it relies on aid to feed its 23 million people, but that has not deterred it from provocations.

Slideshow ( 22 images )

The U.S. Air Force will deploy 12 advanced F-22 Raptor fighters in the coming days to a base in Okinawa, Japan. The move had been planned in advance and was not related to recent rumblings from Pyongyang, a U.S. Forces Japan spokesman said.

The South’s largest newspaper Chosun Ilbo quoted defense sources as saying the South has been preparing for contingencies such as artillery or missile strikes near a contested sea border off the west coast of the peninsula.

A spokesman for the North’s military on Wednesday said the country could not guarantee the safety of the South’s vessels in those Yellow Sea waters that have been the site of deadly naval skirmishes between the states in 1999 and 2002.

The spokesman also said South Korea’s decision to join a U.S.-led anti-proliferation initiative this week was a declaration of war making the armistice that ended the 1950-53 Korean War invalid. Its military would also attack if the South inspects its ships.

Seoul’s financial markets, which had fallen in the wake of the nuclear test, rose on Thursday although traders said investors were still nervous about what further steps the North might take to raise tension in the economically powerful region.

North Korea kept up its steady string of strident rhetoric, saying in its official media that “a minor accidental clash could lead to nuclear war.”

“As circumstances show, provocations of war on the part of the U.S. and South Korea have gone well beyond the risky level. It’s a matter of time when a fuse for war is triggered,” the North KCNA news agency reported a commentary in a state newspaper as saying.