Mattis told Japanese Prime Minister Shinzo Abe during a trip to Asia earlier this week that the Trump administration would continue the longstanding us policy of honoring Tokyo's claim to the Senkaku islands, known as the Diaoyu in China.
"I think the Abe administration is now relieved", said Mikio Haruna, a noted journalist who specializes in diplomacy and is a visiting professor at Waseda University in Tokyo.
Prime Minister Abe was the first to extend a hand of friendship sitting face to face with Mattis he said: "I do have hope and I am convinced that together with you, we will be able to demonstrate the unwavering alliance between Japan and the United States both to the public inside and outside Japan".
Lu said the so-called U.S. -Japan treaty of mutual cooperation and defense is a product of the Cold War, which should not impair China's territorial sovereignty and legitimate rights.
Article 5 of that treaty says the U.S. will defend territories under Japanese administration, which would include the Senkakus, an uninhabited group of islands in the East China Sea called the Diaoyus by China.
Beijing and Tokyo are involved in sovereignty disputes over the Diaoyu Islands, known as Senkaku in Japan.
On Friday, Mattis told journalists during his visit to Tokyo, to meet with defense officials there that a treaty between the two countries also covered the islands.
In the next high-level talks, Japan may be asked to expand its own role to contribute to bilateral security ties, Michishita said.
"If it is the trade surplus that Trump cares the most about, for instance, then we could come up with a few possible solutions", including importing more USA shale oil or gas.
Earlier in the week, Mattis issued similar reassurance to South Korea and warned North Korea that any use of nuclear weapons would be met with an "effective an overwhelming response".
Inada said the costs have been "appropriately shared based on the agreement between the two countries".
"As our alliance grows", Mattis said, "it will be important for both of our nations to continue investing in our defense personnel and capabilities".
On the campaign trail, Trump raised the possibility of Japan and South Korea arming themselves with nuclear weapons, and accused Seoul and Tokyo of not paying their fair share for United States troops stationed in their countries.
U.S. President Donald Trump has accused allies of freeloading under the U.S. security umbrella.
China also registered its displeasure with Mattis' remarks Friday in South Korea that Trump's administration is committed to carrying through on a deal the Obama administration reached with the Seoul government last year to deploy a high-end US missile defense system to South Korea this year.
Mattis also said that a clause written in Japan's Constitution was as relevant today, as it was when it was first written.
A high-ranking Foreign Ministry official, speaking on condition of anonymity, said the reassurance by Mattis bears "political importance", especially when various diplomatic policies of Trump's administration remain unclear.Japanese officials had been concerned that Mattis might demand Japan shoulder more of the cost of keeping USA military forces stationed in Japan, as Trump repeatedly demanded during his election campaign.
The secretary said he also expressed to Inada the United States' appreciation for Japan's stabilizing and strengthening efforts with its Southeast Asian partners, which contribute to regional peace, prosperity and freedom. Tomomi Inada that her country had "been a model of cost sharing". And in any case, while USA allies favor a stout American presence in the region, they have no appetite for military action.
A softer line taken by US Defence Secretary Jim Mattis over the South China Sea on Saturday may help clarify Washington's stance, but Beijing is likely to remain very cautious on the rhetoric of President Donald Trump's administration, Chinese analysts said.
"We urge the U.S. side to take a responsible attitude, stop making wrong remarks. and avoid making the issue more complicated and bringing instability to the regional situation", Foreign Ministry spokesman Lu Kang said, according to official news agency Xinhua.
Once seen as a panacea for Japan's energy crisis after the Fukushima nuclear disaster in 2011 led to the shutdown of most reactors in the country, US shale gas is now just one of many options for Japan to meet its needs.