By Arshad Mohammed

BISHKEK, Kyrgyzstan (Reuters) - The United States wants China to deliver a "tough" message to North Korea to rein in its nuclear program and belligerent rhetoric, a U.S. official said on Friday as Secretary of State John Kerry flew to Asia.

Kerry begins a three-day visit to Seoul, Beijing and Tokyo as U.S. and South Korean officials say the nuclear-armed North appears poised to test a medium-range missile after weeks of threatening statements.

Making his first trip to the region since taking office on February 1, Kerry hopes both to persuade China to use its economic and diplomatic leverage to try to temper North Korea's behavior and to reassure U.S. allies Japan and South Korea.

"We really want them to ... carry some tough messages to Pyongyang" on denuclearization, a senior U.S. official told reporters traveling with Kerry, who arrives in Seoul on Friday, in Beijing on Saturday and in Tokyo on Sunday.

Kerry also wanted to convey an unambiguous message in Seoul and Tokyo that "we are prepared and that alliance matters (and) that we will defend them," the official, who spoke to reporters on condition of anonymity, added.

North Korea carried out its third nuclear test in February, prompting a fresh U.N. Security Council sanctions resolution against the impoverished state led by Kim Jong-un, the 30-year-old grandson of its founder.

Pyongyang's bellicose statements have markedly increased since the sanctions were approved.

As a result, the United States has found itself in the familiar position of trying to get China, which is the North's main economic partner and the closest thing it has to a diplomatic ally, to put more pressure on its neighbor.

"It is no secret that China has the most leverage, the most influence with North Korea," said a second U.S. official. "We would want them to use some of their leverage because otherwise, it is very destabilizing and, in the end, it threatens the whole region."

The first official said the United States wanted China to take some tangible actions, including rigorously carrying out U.N. Security Council resolutions requiring it to crack down when there is a "money trail" leading to North Korean entities involved in weapons of mass destruction.

Pyongyang has threatened a nuclear strike on the United States - although U.S. experts do not believe it has the capability to reach the continental United States - and "war" with "puppet" South Korea, threats that appear at least in part at shoring up internal support for 30-year-old leader Kim Jong-un.

SHIFT IN CHINESE RHETORIC

South Korean and U.S. officials believe it is preparing to launch a Musudan missile, whose range of 3,500 km (2,100 miles) or more, according to South Korea, would put Japan within striking distance and may threaten Guam, home to U.S. bases.

The North is also angry about weeks of joint South Korean-U.S. military exercises. About 28,000 U.S. forces are permanently based in South Korea.

Most observers say Pyongyang has no intention of igniting a conflict that could bring its own destruction but see a risk of miscalculation on the highly-militarized Korean peninsula.

U.S. officials acknowledged the limits of their influence over and information about the North, one of the world's most closed and isolated states, saying they were grappling with what was motivating Kim or whether he was indeed calling the shots.

They also said they believed that China's rhetoric on North Korea has begun to shift, pointing to a speech by Chinese President Xi Jinping in which - without referring explicitly to Pyongyang - he said no country "should be allowed to throw a region and even the whole world into chaos for selfish gain."

"China is increasingly concerned about the downstream effects of North Korea's reckless pursuit of a nuclear missile capability and the implications for China's own strategic environment," said the second U.S. official.

"We all hear a growing tone of frustration and urgency in the official statements from the Chinese," he added.

(Reporting By Arshad Mohammed; Editing by Cynthia Osterman)