In South Korea, Secretary of State Rex Tillerson said the U.S. would not rule out use of force to confront a growing nuclear threat from North Korea.

The last two times North Korea fired off missiles – a multiple launch earlier this month and a more advanced model in February – U.S. intelligence was caught by surprise. North Korea’s increasing ability to launch without warning is one reason Secretary of State Tillerson overturned two decades of U.S. policy toward North Korea.

U.S. Secretary of State Rex Tillerson, left, looks on South Korean Foreign Minister Yun Byung-se during a press conference in Seoul Friday, March 17, 2017. Tillerson visited the world’s most heavily armed border, greeting U.S. soldiers on guard near the tense buffer zone between rivals North and South Korea. Jung Yeon-Je/Pool Photo via AP

“Let me be very clear: The policy of strategic patience has ended. We are exploring a new range of diplomatic, security and economic measures,” Tillerson said Friday after visiting the heavily militarized border between the rival Koreas.

Strategic patience is the diplomatic way of saying the U.S. has relied on economic sanctions, covert sabotage, and cyber attacks to make it difficult for North Korea to develop nuclear weapons.

But as President Trump put it in a tweet: “North Korea is behaving very badly. They have been ‘playing’ the united states for years.”

Following a visit to the DMZ dividing the two Koreas, Secretary Tillerson said in effect “no more games” -- and raised the threat of a pre-emptive strike against the north.

“If they elevate the threat of their weapons program to a level that we believe requires action, that option is on the table,” he said.

But a preemptive strike would almost certainly result in massive loss of life by triggering a North Korean artillery barrage on the South Korean capital of Seoul.

Secretary Tillerson will travel next to Beijing in an effort to convince President Xi to use his country’s economic leverage to pressure North Korea into giving up its weapons program.

But, according to the president’s tweet, “China has done little to help.”

Secretary Tillerson said “all options are on the table.” But a Defense Dept. official added “none of them are good options.”