The vice president met Japanese officials, including Prime Minister Shinzo Abe, and reassured them of the United States commitment in the face of possible threats posed by North Korea.
China's air force has put its land-attack, cruise-missile-capable bombers "on high alert" in response to a potential situation in North Korea, a U.S. defence official has been quoted as saying.
"We are exploring a new range of diplomatic, security, and economic measures and all options are on the table", Gary Ross, a Pentagon spokesman, said.
When asked if he had ruled out some sort of military strike against North Korea, Trump said he did not wish to reveal his plans.
UN Secretary-General Antonio Guterres told reporters that the United Nations fully backs efforts of all states trying to ensure "that North Korea doesn't acquire the capacities that would become a threat, not only for the region but in a wider area of the world". "They are afraid of the firm alliance between South Korea and the USA, and of strong independent defense capabilities".
The three met in Washington in February.
Trump, in Washington, and Pence at the tense Demilitarized Zone between North and South Korea earlier, have signaled a forceful US stance on North Korea's recent actions and threats. Another U.S. official said the alert status was not restricted to one particular region of China but spread out across the country.
An official at the Russian embassy reminded that these words were a quote from the last resolution on North Korea, which was unanimously adopted by the Security Council in 2016.
Mattis struck an even tougher tone on North Korea in an interview with reporters traveling with him to Saudi Arabia.
He was optimistic that China would be able to use its influence on North Korea and its leader Kim Jong Un, who he hoped is "mentally stable".
He said the USA wanted peace and he hoped North Korean ruler Kim Jong Un wanted the same, adding that would be "the end determination".
In Tokyo, there is still hope that the core of the agreement, thrashed out between the United States and Japan and meant to counterbalance China's regional economic power, can be salvaged in some form.
Nikki Haley, the U.S. ambassador to the United Nations, said Wednesday that the USA would not seek to escalate tensions with North Korea and wanted to foster a peaceful solution.
The U.S. and South Korea are holding joint military exercises on the Korean peninsula.
For its part, China made a plea for a return to negotiations.