Japan has promised Vietnam ships to strengthen its forces in the South China Sea, with the two nations describing large-scale land reclamation there as a threat to peace -- a veiled reference to China.
Prime Minister Shinzo Abe also pledged on Tuesday some $835 million in infrastructure loans after holding talks with Nguyen Phu Trong, in his first visit to Japan as general secretary of Vietnam's ruling Communist Party.
Both countries are locked in separate maritime territorial disputes with an increasingly assertive China and are strengthening cooperation as a result.
Related: The scramble for the South China Sea
A statement after the talks said the leaders "expressed their serious concerns over recent and ongoing developments in the South China Sea", although they stopped short of referring directly to Beijing.
These "large-scale land reclamation and building of outposts have increased tensions, eroded trust and confidence, and threatened peace and stability in the region and the world", they said.
China claims the right to almost all of the South China Sea, parts of which are also claimed by the Philippines, Brunei, Malaysia, Taiwan and Vietnam.
In recent months, satellite images released by the US showed China reclaming a vast portion of Spratlys in South China Sea, which could accommodate military planes.
Japan and China are locked in conflict over disputed islands in the East China Sea.
Abe pledged to give more second-hand vessels to Vietnam but did not specify how many.
Japan's foreign minister last year said his country would give Hanoi six used ships for patrols in the South China Sea.
"Japan decided to give Vietnam additional second-hand vessels at its request," the Japanese premier told a news conference.
"This decision will benefit Vietnam to improve its maritime law enforcement capability."
Trong said he and Abe "shared concerns over the complex situations in the East China Sea and the South China Sea".
Vietnam has been trying to strengthen ties with other nations to counter China's growing assertiveness.
In July Trong was received by US President Barack Obama in the Oval Office, where he said they discussed activities in the South China Sea "that are not in accordance with international law".
Vietnam and the Philippines -- the two Southeast Asian nations most vocal in their criticism of China -- this month pledged to sign a "strategic partnership" agreement to bolster defence, political and economic ties.
Japan has been trying to increase its influence in the region, and in July pledged $6.1 billion in financial aid to the "Mekong Five" countries of Cambodia, Laos, Myanmar, Thailand and Vietnam.