Kerry heads to S.Korea as tensions soar

US Secretary of State John Kerry was headed for South Korea Thursday on his maiden trip to Asia in a show of American support for regional allies as tensions soar over a feared new missile test by the North.

The volatile situation on the Korean peninsula fuelled by a series of bellicose statements by Pyongyang was one of the global crises crowding the agenda for Kerry's two-day talks in London with G8 foreign ministers.

North Korea has threatened nuclear strikes against the United States and South Korea, and observers expect a missile launch could happen at any time.

The G8 foreign ministers condemned North Korea's nuclear moves "in the strongest possible terms", they said in a statement ahead of Kerry's departure from Britain.

Pyongyang was Thursday marking the first anniversary of new leader Kim Jong-Un taking over as head of the ruling Worker's Party, and any launch could be timed to seize the limelight as Kerry arrives on the divided peninsula.

Monday will also be the birthday of Kim's grandfather and the late founder of the communist state Kim Il-Sung -- an occasion which is usually marked with lavish celebrations.

Kerry, who only took over as the top US diplomat on February 1, has denounced Pyongyang's actions, including a threat to reopen an uranium enrichment facility, as "provocative."

The United States has already bolstered its missile defenses in the region to help protect allies South Korea and Japan as well as US bases in Guam, while US and South Korean forces are on alert.

Kerry is due to be briefed first-hand on the tensions when he arrives in Seoul from top US military commanders on the ground, ahead of meetings with new South Korean President Park Geun-Hye and Foreign Minister Yun Byung-Se.

"North Korea... with its bellicose rhetoric, its actions, has been skating very close to a dangerous line," US Defence Secretary Chuck Hagel has said.

Kerry's whirlwind three-day trip is his first to Asia since becoming secretary of state and he will also make stops in China and Japan on what is being seen by observers as a "getting to know you" tour.

Despite being a veteran of the Vietnam war and a long-time member of the Senate foreign relations committee, he has not travelled much to the region in recent years and does not have close ties with Asian leaders.

Saturday's visit to Beijing will be key, with Kerry expected to try to press Chinese leaders including Chinese Foreign Minister Wang Yi to do more to rein in their ally and neighbour, North Korea.

Washington "would hope that China would do a lot of things to restrain the flow of energy and food to North Korea... hold back on new investment," said expert Douglas Paal from the Carnegie Endowment for International Peace.

The problem is that "those all run against China's primary interest to North Korea which is to provide for stability there."

But with a new level of debate being seen in China about what to do with North Korea, "it's worth putting our views forward and hope to tilt the debate in our direction," Paal added in a recent conference call.

As all sides seek to stop the situation from spiralling out of control, experts also say it might be time to revisit the 1953 armistice which ended the Korean War with a ceasefire pact, rather than a full peace treaty.

Carnegie expert James Schoff called the armistice "tattered and frayed on the edges." Much could be done to "update the armistice" and help "reduce tension or the potential for conflict or accidental conflict on the DMZ," he said.

Kerry's final stop will be in Japan for talks on Sunday and Monday, before flying back to Washington.