On Tuesday, Mattis said the United States is "working so closely" with China towards the aim of denuclearizing the peninsula, adding hopefully that: "We all share that same interest".
He also said the U.S. will honor its alliance with Pacific Rim nations to protect freedom of navigation in the disputed South China Sea - most of which Beijing lays claim to, despite conflicting claims from other Asian nations.
On April 15, the U.S. Navy published a photo showing the aircraft carrier USS Carl Vinson transiting the Sunda Strait between the Indonesian islands of Java and Sumatra.
She said the Chinese have "gotten the message about increasing pressure and that they have said they want to increase cooperation with us and with the global community on this front".
France said on Wednesday it was imperative the United Nations Security Council apply more pressure on North Korea to prevent it jumping another hurdle towards an effective nuclear weapons capacity that could lead to a risky military escalation. The North Korean diplomat accused Washington of creating "a situation where nuclear war could break out at any time" and that Pyongyang's next nuclear test would take place "at a time and at a place where our headquarters deems necessary".
State television aired footage of a choral performance attended by Kim Jong-un, the elder Kim's grandson, on Sunday, a day after a huge military parade in Pyongyang also dedicated to the 105th birth anniversary of Kim Il-sung.
Beijing has signed up to United Nations sanctions against North Korea's missile and nuclear programs, and in February vowed to halt coal imports, but trade and finance continue. North and South Korea are technically still at war because their 1950-53 conflict ended in a truce, not a peace treaty. "I think it's important to see them heading in this direction".
Pence has met with Japanese Prime Minister Shinzo Abe.
The United States kicked off economic talks with Japan yesterday, with an eye on eventually brokering a bilateral trade pact despite the latter's preference for multilateral deals.
He was on more solid ground with the North Korean issue, saying: "We appreciate the challenging times in which the people of Japan live with increasing provocations from across the Sea of Japan". "We are with you 100 percent".
He also said it was reassuring that Mr Pence had not cited trade reforms as a pre-condition for the US-South Korea military alliance.
He said that President Donald Trump's administration will continue to "work diligently" with other countries to apply economic and diplomatic pressure on Pyongyang, and that the United States "will always seek peace".
The Reuters report also cited a statement from Susan Thornton noting that Secretary of State Rex Tillerson and China's top diplomat, State Councilor Yang Jiechi, agreed in a phone call on April 16 on the need for strict enforcement of United Nations resolutions.
Chinese Foreign Minister Wang Yi on Tuesday made a new appeal calling for calm on the Korean peninsula.
"The President is determined to keep our promise and I'm very confident that in the days ahead, we're going to see the Congress come together and we're going to take that important first step to repeal and replace Obamacare with the kind of health care plan President Trump has envisioned", he told CNN's Dana Bash Wednesday.