Kerry started what will be a three-day stay in the region in Laos, the 2016 chair of the 10-member Association of Southeast Asian Nations (ASEAN). He will head to Cambodia on Monday night and then on to Beijing for talks on Wednesday with the leadership there.

In Beijing, Kerry is expected to stress the need for a united front in response to this month`s North Korean nuclear test through additional U.N. sanctions, a senior official of the U.S. State Department said. He will also argue for a tough unilateral response from China, North Korea`s main ally and

neighbor.

"It is very important to present a united front ... but that united front has to be a firm one, not a flaccid one," the official told journalists traveling with Kerry.

It was particularly important to "cut off avenues of proliferation and retard North Korea`s ability to gain the wherewithal to advance its nuclear and its missile programs," the official said, and that meant China doing more.

North Korea said on Jan. 6 it had successfully tested a hydrogen bomb, although Washington voiced skepticism as to whether the device was actually that powerful.

"North Korea is still engaged in illicit and proliferation activities," the official said. "They have very few avenues for conducting business with the international community that don`t in some fashion involve transiting China.

"Despite the determination and efforts of the Chinese government, clearly there is more that they can do."

In Beijing Kerry plans "in depth" discussions on the South China Sea, a source of increasing tension between China and ASEAN countries and the United States due to China`s building of artificial islands suitable for use as military bases, the official said.

CHINESE ALLY

First though in Vientiane, the capital of Laos, Kerry will seek to bolster ASEAN unity and the bloc`s resolve to stand up to China in the lead-up to a summit President Barack Obama has called with the bloc`s leaders for Feb. 15-16 in Sunnylands, California.

Laos has close political and economic ties with its giant neighbor China. The Obama administration worries that it might behave as Cambodia did when it held the ASEAN chair in 2012 and was accused of obstructing consensus in the bloc over the South China Sea.

Besides its China ties, as a landlocked country Laos has less interest in the maritime disputes that several ASEAN members have with Beijing.

The U.S. official said he had heard from virtually every ASEAN country that the Cambodian chairmanship had left "a black mark" on the bloc that was not to be repeated.

So far, Laos was off to a good start overseeing ASEAN statements on world events, the official said, adding: "It`s my expectation that the Lao will be a responsible chair for 2016."

Kerry will seek to set an encouraging tone in Laos by discussing increased U.S. aid, including more funding for work to dispose of unexploded U.S. ordnance left over from the Vietnam War. During that conflict Laos became one of the most heavily bombed countries in history as the United States sought to destroy communist supply lines running through it.

The main announcements, though, are expected to come when Barack Obama attends a regional summit towards the end of the year and becomes the first U.S. president ever to visit Laos.

In Cambodia, Kerry will meet Hun Sen, now Asia`s longest serving prime minister, and will draw attention to U.S. concerns about human rights and treatment of government critics by meeting opposition members and civil activists, the State Department official said.

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