US Defense Secretary Jim Mattis reportedly said on Friday that President Donald Trump's administration, like predecessor Barack Obama's, puts China's Diaoyu Islands under the United States security umbrella offered to Japan.

He made the remark during his first trip abroad after taking office in January, according to Japan's state broadcaster NHK. Mattis visited South Korea before arriving in Tokyo, and he talked to Japanese Prime Minister Shinzo Abe on Friday afternoon.

Mattis reassured Abe that the islands fall within the scope of Article 5 of the US-Japan Treaty of Mutual Cooperation and Security, according to NHK, which quoted unnamed Japanese officials.

The article states that either country would act to meet the common danger in case of "an armed attack against either party in the territories under the administration of Japan".

According to Reuters, Mattis said the article "is understood to be as real to us today as it was a year ago, five years ago, and it will be a year and 10 years from now".

As of press time, the US had not confirmed Mattis' remark about the islands.

During Abe's trip to Washington in April 2014, Obama became the first US president to clearly assure Japan that the islands are covered by the article.

Yang Bojiang, a senior researcher on Japan studies at the Chinese Academy of Social Sciences, said Tokyo's pursuit of US reassurance is not a wise choice, since Japan also says it seeks to improve its chilly ties with China.

Yang said that seeking to reaffirm security pledges is a tradition of Tokyo when power changes hands in Washington, and the reassurance this time is more like a "symbolic pledge", since the current status of China-Japan ties indicates that the chance of a major conflict breaking out around the islands is small.

Since Abe is scheduled to meet with Trump in the US later this month, the reaffirmation "is part of a 'give and take' game of the Trump administration to remind the ally to do something in return, particularly on economic and trade issues", Yang said.

Zhao Xiaozhuo, a researcher at the Center on China-US Defense Relations at the PLA Academy of Military Science, said the reassurance "signals that Washington's support for treaty allies will not shrink as Trump had pledged on the campaign trail" and will ease Japan's anxiety.

"For China, the latest pledge is more like a test," Zhao said. "Trump's Asia-Pacific policy is still taking shape, and his policy is likely to follow Obama's tune."