US Vice President Mike Pence is expected to reassure Japan of American commitment to reining in North Korea's nuclear and missile ambitions on Tuesday, after warning that U.S. strikes in Syria and Afghanistan showed the strength of its resolve.
Pence took off for Tokyo from a U.S. air base south of Seoul, where he assured leaders of the "iron-clad" alliance with the United States. He also warned the reclusive North, which has conducted a series of missile and nuclear tests in defiance of U.N. sanctions, that the "era of strategic patience" was over.
He is expected to discuss Korean tensions with Japanese Prime Minister Shinzo Abe, as well as hold economic talks with Finance Minister Taro Aso.
North Korea regularly threatens to destroy Japan, South Korea and the United States and it showed no let-up in its defiance after a failed missile test on Sunday, a day after putting on a huge display of missiles in Pyongyang.
North Korea's deputy representative to the United Nations, Kim In Ryong, accused the United States of creating "a situation where nuclear war could break out an any time" and said Pyongyang's next nuclear test would take place "at a time and at a place where our headquarters deems necessary".
North Korea's Vice Foreign Minister Han Song-Ryol told the BBC that missiles would continue to be tested on "a weekly, monthly and yearly basis".
South Korea's acting president, Hwang Kyo-ahn, told a cabinet meeting on Tuesday that South Korea would strengthen its alliance with the United States and closely cooperate with China to rein in North Korea.
"We should stay on our toes to protect our territory and people's lives," Hwang said.
The North has warned of a nuclear strike against the United States if provoked. It has said it has developed a missile that can strike the mainland United States, but officials and experts believe it is some time away from mastering the necessary technology, including miniaturizing a nuclear warhead.
Pence said on Monday the world had witnessed U.S. President Donald Trump's resolve in the past two weeks, which saw a U.S. missile attack on a Syrian airfield and the dropping of a powerful non-nuclear bomb on Islamic State fighters in Afghanistan.
"North Korea would do well not to test his resolve or the strength of the armed forces of the United States in this region," Pence said.
The Trump administration has said military action remains an option for dealing with North Korea.
But, mindful that this would likely trigger massive retaliation and casualties in South Korea and Japan, U.S. officials say the Trump administration's main focus is on tougher economic sanctions.
Trump, when asked on Monday if he was considering military action, told Fox News Channel he didn't want to "telegraph" his plans like the previous administration.
U.S. officials say tougher sanctions could include an oil embargo, a global ban on North Korea's airline, intercepting cargo ships and punishing Chinese banks doing business with Pyongyang. They say greater Chinese cooperation is vital.
White House spokesman Sean Spicer said China had taken some "very helpful" steps, although it remained to be seen how effective these would be.
Susan Thornton, acting U.S. assistant secretary of state for East Asia, said Secretary of State Rex Tillerson and China's top diplomat, State Councillor Yang Jiechi, agreed in a phone call on Sunday on the need for strict enforcement of U.N. resolutions and for international action to press Pyongyang "to cease provocative actions and recommit to peaceful denuclearization".
Thornton said any new North Korean nuclear test "would draw a pretty significant international response".
Chinese Foreign Minister Wang Yi repeated China's standard line that the crisis could only be resolved by diplomacy.
"I've seen that the United States has reiterated it is willing to use political and diplomatic means to resolve this, as this is their first choice," he told reporters in Beijing.
"Of course I think that any country will feel that political-diplomatic means are of course the first choice."
Pence's economic discussions in Tokyo will be closely watched to see how hard a line Washington is prepared to take on trade. Trump campaigned on an "America First" platform, and has vowed to narrow big trade deficits with nations like China and Japan.
However, Trump has also shown willingness to link trade to other issues, saying he would cut a better trade deal with China if it exerts influence on North Korea.
China banned imports of North Korean coal, its most important export, in February, and Chinese media have raised the possibility of restricting oil shipments to the North.
Impoverished North Korea and the rich, democratic South are technically still at war because their 1950-1953 conflict ended in a truce, not a peace treaty. South Korea hosts 28,500 U.S. troops to counter the threat from the North.