TOKYO - China has built a new pier for warships at a site being developed as a military base near the Japanese-administered, Chinese-claimed Senkaku Islands in the East China Sea, sources close to the matter said Friday.

By building the pier on one of the islands in the Nanji chain off the port city of Wenzhou, Beijing is apparently aiming to improve its preparedness for a possible contingency with Japan and the United States, both critical of China's growing maritime assertiveness, the sources said.

The island chain is about 100 kilometers closer to the Senkakus than the main island of Japan's Okinawa Prefecture, home to a large portion of U.S. military bases in Japan.

China also appears keen to enhance monitoring of the air defense identification zone it declared over the sea in 2013 to fortify its territorial claims there. Neither Japan nor the United States have recognized this zone.

The sources said the pier was constructed on Nanji Island, the largest in the chain, and several warships have already been witnessed docking there. The 70- to 80-meter-long pier can also accommodate landing craft.

The islands already have an advanced radar system in place and a heliport for use by carrier-based helicopters.

Plans are under way to build a runway for military use on the island, but construction does not appear to have started, the sources said. Exercises by military aircraft were held in the area in spring, they said.

Tensions remain high between Japan and China over the Senkaku Islands, uninhabited islets which China calls Diaoyu. Japan is increasingly concerned about Chinese government vessels' repeated entry into Japanese waters near the Senkakus.

In Wenzhou, Zhejiang Province, a plan for the China Coast Guard to build a large-scale base is in the works to boost support for vessels monitoring the Senkakus.

A Japanese national was detained in Wenzhou in May last year and later indicted. Before his detention, the man was purportedly investigating a military facility.

Beijing also has territorial spats with other Asian countries, notably the Philippines and Vietnam, in the South China Sea.

While Japan and the United States are not directly involved in overlapping claims in that sea, they have criticized China's land reclamation work and construction of outposts for military purposes.

==Kyodo