The resource-rich South China Sea is claimed in its entirety by Beijing but is also contested by Vietnam, the Philippines, Malaysia, Brunei and Taiwan
Japan and Vietnam agreed to step up security and defence cooperation on Monday, reaching an agreement in principle for Tokyo to export defence equipment and technology to the Southeast Asian nation.
The deal comes as concerns mount over China's increasing assertiveness in the contested South China Sea, with Beijing expanding its military presence in the region.
The pact would allow Japan to export equipment, likely including patrol planes and radar, to Vietnam, according to Japanese news agency Kyodo.
"It is a big step in the field of security and defence cooperation between the two countries that we reached an agreement in principle on the transfers of defence equipment and technology," said Japanese Prime Minister Yoshihide Suga in Hanoi during his first visit overseas since taking office last month.
The resource-rich South China Sea is claimed in its entirety by Beijing but is also contested by Vietnam, the Philippines, Malaysia, Brunei and Taiwan.
China has reinforced its claim to the waterway by building up small shoals and reefs into military bases with airstrips and port facilities, and it launched ballistic missiles in the flashpoint waters in August as part of live-fire exercises.
They would also work together on the issue of Japanese citizens kidnapped by North Korea, he said.
Japan suspects dozens of people who are still missing were abducted by North Korean agents in the 1970s and 1980s to train their own spies in the Japanese language and culture.
Japan and Vietnam also reached an agreement on starting "business track" flights after travel between the two nations was suspended in March.
This would allow executives and skilled workers to travel without a 14-day quarantine period provided they follow certain Covid-19 precautions, Kyodo said.