Vice President Pence told North Korea on Monday that it should “not mistake the resolve of the United States of America to stand with our allies,” warning that military action is still a possibility to stop Kim Jong Un’s regime from advancing its nuclear and missile programs. 

The vice president delivered the message as he stood just a few feet away from the military demarcation line that separates North and South Korea, a line he described as a “frontier of freedom.”

“The people of North Korea, the military of North Korea, should not mistake the resolve of the United States of America to stand with our allies,” Pence said, describing links with South Korea as “ironclad.”

The vice president repeated the Trump administration line that “the era of strategic patience is over,” referring to the Obama-era policy of putting pressure on North Korea and waiting until it became unbearable.

Pence arrived in South Korea just hours after North Korea launched its latest ballistic missile — which exploded within a few seconds — and amid a weekend of fanfare in North Korea, during which the regime showed off what appeared to be new missiles designed to reach the United States.

North Korea had been expected to do something provocative to mark the most important day on the regime’s calendar, the April 15 anniversary of founder Kim Il Sung’s birthday.

Although the vice president said that the United States wanted to deal with North Korea “through peaceable means, through negotiations,” he made it clear that military force would be used if necessary.

“All options are on the table as we continue to stand shoulder to shoulder with the people of South Korea for denuclearization of this peninsula and for the long-term prosperity and freedom of the people of South Korea,” he said as he stood in a building near the line called Freedom House.

He did not go into any of the blue conference buildings that straddle the line with North Korea. When Secretary of State Rex Tillerson visited South Korea last month and went into those conference buildings, North Korean soldiers stood on the other side of the windows, a few feet away, taking photos of him.

Pence’s remarks could reignite fears that military options for dealing with North Korea are still being considered.

The U.S. Navy rerouted an aircraft carrier strike group to the region earlier this month as tensions were building, and President Trump has repeatedly tweeted that the United States will take action to stop North Korea if China, its closest ally, doesn’t.

Pence said Monday that he is hopeful that China will do more to solve the problem. “We look for them to do more,” he said.

Earlier, a White House foreign policy adviser traveling with Pence said that the United States did not need to take action “to reinforce their failure,” referring to the missile launch. North Korea has fired numerous medium-range missiles, as Sunday’s appeared to be, so one more made little difference.

“If it had been a nuclear test, then other actions would have been taken from the U.S.,” the adviser told reporters on the vice president’s plane.

Some analysts were puzzled by this.  

“I’m not sure why failed missile tests, which are still banned by the U.N. Security Council, are considered less provocative,” said Kent Boydston, a North Korea-focused research analyst at the Peterson Institute for International Economics. “The North Koreans know they may fail, but they improve their capabilities each time.”

Indeed, while the latest missile exploded soon after launch, experts say North Korea is able to learn from its mistakes and hone its technology. It had repeated failures with other missiles, including the medium-range Musudan and a submarine-launched ballistic missile, before successfully firing both.

North Korea’s behavior will be the focus of Pence’s trip to Asia, with a senior administration official saying the vice president would be discussing “the belligerency of North Korea” at every stop. 

After stopping in Seoul, Pence is to travel to Tokyo; Jakarta, Indonesia; and Sydney. 

China on Friday warned that “storm clouds” were gathering. 

Foreign Minister Wang Yi urged the United States and North Korea not to push their recriminations to a point of no return and allow war to break out on the peninsula.

On Sunday, China’s top diplomat, Yang Jiechi, spoke to Secretary of State Rex Tillerson by telephone, and the two “exchanged views on the current situation on the Korean Peninsula,” China’s foreign ministry said in a statement.

China said it cut off coal imports from North Korea in February, in accordance with U.N. Security Council sanctions. 

The Global Times, an English-language Chinese newspaper that does not necessarily reflect official policy, recommended that Beijing convey a clear message to Pyongyang: that if it conducts a sixth nuclear test, the Chinese government would support stiffer U.N. sanctions to cut off “the vast majority” of its oil supplies to North Korea.

The U.S. Navy sent a strike group to Korean waters last week, but defense officials have said tougher sanctions and pressure was still the favored approach to dealing with North Korea.

“We’ve got a range of options, both militarily, diplomatic and others . . . at disposal for the president should he choose to use them,” the White House foreign policy adviser told reporters on Air Force Two. “But for this particular case [of the failed missile launch] . . . we don’t need to expend any resources against that.”

Tensions are expected to continue for several weeks at least. 

Large-scale annual exercises between the South Korean and U.S. militaries will continue until the end of April, and North Korea will mark another important date — the anniversary of the foundation of the Korean People’s Army — on April 25. 

Military action is strongly opposed in South Korea because it would likely bear the brunt of any retaliation. North Korea is thought to have conventional artillery lined up on its side of the DMZ, trained on Seoul, a city of 10 million people just 30 miles from the border.

The fear of devastation in Seoul — and the risk to the American troops based in and around the South Korean capital — has long restrained U.S. administrations from striking North Korea.

Simon Denyer in Beijing contributed to this report.