Japan says it will give Vietnam six naval ships for patrols in the South China Sea, amid regional tension over competing maritime claims with China.
The offer, worth 500 million yen (£2.9 million, $5 million), was announced during a visit by Japanese Foreign Minister Fumio Kishida to Hanoi.
China angered Vietnam when it moved an oil rig to disputed waters in May, sparking deadly protests in Vietnam.
Japan and China both claim a string of islands known as Diaoyu or Senkaku.
The six boats are used vessels and will be accompanied by training and equipment to help coastguard and fisheries surveillance, Mr Kishida told reporters.
He also said that both Vietnam and Japan agree on "maintaining peace and stability" in regional waters, and disputes must be settled "in accordance with international law".
Vietnam's Thanh Nien News quoted China security policy expert Yun Sun as saying Japan's gift could be seen as an "alignment of positions" with Vietnam that is "perceived as hostility by China".
It comes after the Philippines, which is also locked in territorial disputes with China, signed a military pact with the United States in April to increase the latter's troop presence in the country, a move which angered China.
China has repeatedly stressed that it prioritises regional stability and that it favours "peaceful development".
But such claims continue to be met with wariness by neighbouring countries given the rapid build-up of China's presence in disputed waters in recent years.
The moving of the Haiyang Shiyou 981 rig to waters off the coast of Vietnam, near the disputed Paracel Islands, in May triggered a low point in China-Vietnam relations.
Anti-China riots broke out in Vietnam during which angry workers targeted foreign-owned factories in some areas, leaving at least two people dead and dozens injured. Several factories were burned down or damaged.
Government ships from China and Vietnam have clashed near the rig on several occasions, bumping and exchanging water cannon fire.
China eventually moved the rig away in July, a month earlier than planned.
Sino-Japan relations have also soured of late, due to the ongoing dispute over the Diaoyu or Senkaku islands in the East China Sea, with ships from both sides regularly confronting one another.
Japan has named five uninhabited isles in that chain. Releasing their names on Friday, the government said the move was meant to raise public awareness that they belong to Japan, according to news agencies.