By David Brunnstrom and Arshad Mohammed
WASHINGTON, Jan 3 (Reuters) - In three words of a tweet this week, U.S. President-elect Donald Trump vowed North Korea would never test an intercontinental ballistic missile.
"It won't happen!" Trump wrote after North Korean leader Kim Jong Un said on Sunday his nuclear-capable country was close to testing an ICBM of a kind that could someday hit the United States.
Preventing such a test is far easier said than done, and Trump gave no indication of how he might roll back North Korea's weapons programs after he takes office on Jan. 20, something successive U.S. administrations, both Democratic and Republican, have failed to do.
Former U.S. officials and other experts said the United States essentially had two options when it came to trying to curb North Korea's fast-expanding nuclear and missile programs - negotiate or take military action.
Neither path offers certain success and the military option is fraught with huge dangers, especially for Japan and South Korea, U.S. allies in close proximity to North Korea.
The Republican president-elect complained in a separate tweet that China, North Korea's neighbor and only ally, was not helping to contain Pyongyang - despite China's support for successive rounds of U.N. sanctions against Pyongyang.
While many critics, including within President Barack Obama's administration, agreed China could press North Korea harder, the State Department said it did not agree with Trump's assessment that China was not helping.
Experts said Trump's tough stance toward Beijing on issues from trade to Taiwan could prove counterproductive in securing greater Chinese cooperation.
James Acton, co-director of the Nuclear Policy Program at Washington's Carnegie Endowment for International Peace think tank, said that with his North Korea tweet, Trump had drawn a red line he could later be judged by, like Obama's 2012 warning to Syria over the use of chemical weapons.
"This was a foolhardy tweet for Trump to send given the enormous challenges of constraining North Korea's nuclear and missile programs. I think this could be something that comes back to haunt him."
THREE OPTIONS
U.S. officials, who did not want to be identified, said that if ordered, the U.S. military had three options to respond to a North Korean missile test - a pre-emptive strike before it is launched, intercepting the missile in flight, or allowing a launch to take place unhindered.
One official, who did not wish to be named, said there were risks with pre-emptive action, including the possibility of striking the wrong target - or North Korean retaliation against regional allies.