Japan has sent two boats to Vietnam to be used as patrol boats amid China's recent demonstrations of force in the South China Sea.

Kyodo News reported Tuesday that the Japanese government had provided two second-hand fishing boats to the Vietnamese coast guard to strengthen the country's law enforcement capability.

China continues to defend its maritime boundary claims in the South China Sea in response to United States warships sailing near the Spratly Islands -- where it has reportedly constructed two artificial islands on Subi and Mischief reefs.

China's military released photos late Sunday that show fighter jets flying over the sea near Vietnamese waters.

Vietnam, Brunei, Malaysia, Taiwan, and the Philippines have also laid claims to areas of the Spratly archipelago.

The donated vessels form part of an aid deal signed between Japan and Vietnam in Aug. 2014.

They were delivered to Danang, a city on Vietnam's east coast, to be refurbished as patrol boats.

The aid-in-kind deal provided for the delivery of six vessels to the Vietnamese side, with two remaining to be delivered, Kyodo News reported.

The U.S. and Japan have expressed alarm at China's maritime expansion, which they suspect is aimed at extending its military reach, while the Philippines -- which calls the area the West Philippine Sea -- has taken the quarrel to the International Tribunal for the Law of the Sea in The Hague.

Japan will reportedly assist Vietnam with additional vessels under a separate agreement signed in Sept. 2014.