Japan will build six patrol vessels for Vietnam to "boost Hanoi's maritime law enforcement capabilities", Japanese authorities said in a report that prompted an expert to caution Tokyo against stoking tensions in the South China Sea.

According to the statement, the Japan International Cooperation Agency, or JICA, signed the agreement with the Vietnamese government in Hanoi on July 28. Under the arrangement, Japan will provide a 36.6 billion yen ($345 million) loan for Vietnam to buy the vessels.

Yu Qiang, a researcher of Japanese studies at the University of International Relations in Beijing, said: "Countries not in the region should refrain from stirring up trouble and creating tensions in the South China Sea, because it is against the common aspiration of countries directly involved to pursue peace and development."

The vessels are due to be delivered by October 2025. Observers said they will likely be based on the Aso-class format, at 79 meters long with a top speed of more than 30 knots. The observers envisaged that the vessels would be fitted with a 40-millimeter main gun and four water cannons－fixtures already in use on boats run by the Japanese Coast Guard.

The JICA said the deal will contribute to "the realization of a free and open Indo-Pacific", phrasing used by the United States when inciting tensions between China and other countries in the South China Sea region.

Yu said the so-called freedom of navigation issue is not a problem caused by China but by non-regional powers like the US.

"China and countries in the South China Sea enjoy good friendship and trust," he said. "It is believed that they can solve their disputes properly as they all advocate peaceful settlement through legal means. However, some other countries are trying to undermine regional solidarity out of geostrategic considerations, and countries in the region should be on high alert about this."

Attempts to incite tensions in the region are not welcomed as the Philippines, for one, has rejected participation in maritime drills led by the US, Yu said.

Philippine Defense Secretary Delfin Lorenzana said Manila would not join the navies of other countries in maritime drills in the South China Sea for fear of raising tensions in the area.

In a meeting with Vietnamese officials in July, State Councilor and Foreign Minister Wang Yi said the South China Sea is the "common home" of China and members of the Association of Southeast Asian Nations and both sides wish to promote regional peace and development.

Vietnamese Deputy Prime Minister and Foreign Minister Pham Binh Minh told Wang that Vietnam is willing to work with China to jointly maintain peace and stability in the South China Sea and promote maritime cooperation and common development.