By Phil Stewart and Kiyoshi Takenaka

TOKYO (Reuters) - U.S. President Donald Trump's defense secretary reaffirmed America's commitment to its mutual defense treaty with Japan on Friday when he met Prime Minister Shinzo Abe in Tokyo.

Jim Mattis, on his first trip since taking over the Pentagon, appeared eager to reassure Japan of U.S. resolve, after a 2016 election campaign in which Trump suggested both South Korea and Japan were benefiting from a U.S. security umbrella without sharing enough of the costs.

Abe is due to meet Trump for talks in the United States on Feb. 10.

Mattis said provocations by North Korea, which is advancing its nuclear weapons and missile programs, left no room for doubt about U.S. commitment. It was similar to a message he delivered over the past two days in South Korea.

"I want to make certain that Article 5 of our mutual defense treaty is understood to be as real to us today as it was a year ago, five years ago - and as it will be a year, and 10 years, from now," said Mattis, a retired Marine general.

Article 5 obliges the United States to defend territories under Japanese administrative control.

Japan had been keen for assurances that Trump's administration will adhere to Washington's commitment to defend disputed East China Sea islands that are under Japanese control but claimed also by China.

A Japanese government statement, issued after the meeting, said Mattis told Abe that Article 5 of the security treaty applied to the contested islands and that the United States would oppose any unilateral action aiming to hurt Japan's administration over the islets.

Patrol ships and fighter jets from Japan and China routinely shadow each other near the islands, called the Senkaku in Japanese and the Diaoyu in Chinese.

A Pentagon statement said Trump's administration would not alter U.S. policy.

China's Foreign Ministry said the islands were Chinese, calling the treaty a product of the Cold War.

"We urge the U.S. side to take a responsible attitude, stop making wrong comments on the issue of the Diaoyu Islands sovereignty, and avoid further complicating the issue and bringing instability to the regional situation," it said.

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Abe said he was convinced that, with Trump and Mattis, the United States and Japan could demonstrate to the world their "unwavering alliance".

He also told Mattis Japan intends to bolster its defense and to "expand the role it can play," according to the Japanese statement.

Japanese Foreign Minister Fumio Kishida echoed that message to Mattis in a meeting later on Friday, saying it was important to further strengthen the alliance in the face of an "increasingly severe" security environment in the region.

Japan's defense spending remains about 1 percent of gross domestic product, far less than that of China.

Mattis is due to hold talks on Saturday with Defense Minister Tomomi Inada, who has repeatedly said Japan is bearing its fair share of the costs for U.S. troops stationed there and has stressed that the alliance is good for both countries.

Since taking office on Jan. 20, Trump has jolted the region by pulling the United States out of an Asia-Pacific trade deal that Japan had championed.

Japan is putting together a package it says could generate 700,000 U.S. jobs and help create a $450-billion market, which will be presented to Trump, government sources familiar with the plans said.

(Reporting by Phil Stewart and Kiyoshi Takenaka in Tokyo, additional reporting by Ben Blanchard in Beijing; Editing by Clarence Fernandez, Robert Birsel and Howard Goller)