Just ahead of Pence's arrival in Seoul, the North attempted and failed to launch an unidentified missile in its latest weapons test, further raising worldwide concerns.
The missile fired on Sunday blew up nearly immediately, the Pentagon said.
For now, both sides seem keen to downplay potential for conflict.
North Korea's deputy United Nations ambassador Kim In Ryong said during a news conference that the US was turning the Korean Peninsula into "the world's biggest hotspot" and creating "a risky distuation in which a thermonuclear war may break out at any moment".
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 United Nations resolutions and for global action to press Pyongyang "to cease provocative actions and recommit to peaceful denuclearisation".
Kim Jong Un showcased his new missiles during a parade last week.
Mr Mattis said Sunday's missile test had not involved an intercontinental ballistic missile but had still been reckless.
"To some extent, we want to do for the United States what we did for IN", the White House adviser said. "In the middle of him talking with North Korea I'm going to hit them with currency manipulation?"
Without looking, can you say the name of the supreme leader of North Korea?
Despite its decision to restrict coal imports, Chinese trade with North Korea has increased.
Despite knowing that the test was a failure, both the South Korean and US government are still unable to determine the type of missiles that North Korea used. And by that I mean, if they can put a warhead on top of a missile that can reach the United States, that's obviously a direct threat.
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Pence took off for Tokyo from a US air base south of Seoul, where he assured leaders of the "iron-clad" alliance with the United States.
President Donald Trump, like his predecessor, Barack Obama, has pressed China to use its political and economic leverage over North Korea, its once-close ally. He also warned North Korea that "they have to behave".
Pence on Monday had traveled to the tense zone dividing North and South Korea, where he warned North Korea's leaders that after years of testing the US and South Korea with its nuclear ambitions, "the era of strategic patience is over". Beijing has retaliated against South Korean businesses, while its military has threatened to take unspecified action in response.
"North Korea's Deputy Foreign Minister Sin Hong-chol responded in kind, saying if we notice any sign of assault on our sovereignty, our army will launch merciless military strikes against the USA aggressors, wherever they may exist".
He has sent a navy strike group towards the Korean Peninsula, and the USA and South Korea are moving ahead with the early deployment of a controversial missile defence system.
So Pence struck a stern tone after arriving at a US naval base from South Korea.
"We hope to achieve this objective (the North's denuclearisation) through peaceful means but all options are on the table", Mr Pence told a press conference in the South Korean capital after his trip to the tense border with the North.
Abe is to discuss North Korea today with Pence, who flew to Tokyo.
"China is not the culprit of the issue nor does it hold the crux and the key to resolving it", Chinese Foreign Minister Wang Yi argued Monday, emphasizing that the USA will decide whether or not there is peace on the peninsula.
In Moscow, Russian Foreign Minister Sergey Lavrov, speaking to reporters Monday evening, said he hopes "there will be no unilateral actions like those we saw recently in Syria".