The Philippines has built a new coast guard station on the contested island of Thitu in the South China Sea, boosting its ability to monitor movements of Chinese vessels and aircraft in the busy disputed waterway.

As tension mounts over territorial claims in the area, the Philippine coast guard had early this year spotted a Chinese navy ship and dozens of militia vessels around the island, one of nine features Manila occupies in the Spratly archipelago.

Inaugurated on Friday, the new three-storey facility is equipped with state-of-the art technology such as radar, automatic identification, satellite communication, and coastal cameras, the Philippine coast guard said in a statement.

"The behaviour of the Chinese coast guard, People's Liberation Army (PLA) Navy and Chinese militias are sometimes unpredictable," Eduardo Ano, the Philippine national security adviser, said during a visit to the island.

"They do not adhere to the international order, to the rule of law," he told reporters on Friday.

"What they're describing as gray-zone tactics ... are pure bullying and it's purely illegal. It's not acceptable in the international order."

The Chinese embassy in Manila did not immediately respond to a request for comment.

Manila's outpost of Thitu is its biggest and most strategically important in the South China Sea, largely claimed by Beijing, despite conflicting territorial claims by several regional nations.

Known locally as Pag-asa, Thitu lies about 300 miles (480 km) west of the Philippine province of Palawan. Home to about 200 people, it is used by Manila to maintain its territorial claim.

Besides the Philippines, Brunei, China, Malaysia, Taiwan and Vietnam have competing claims of sovereignty in the South China Sea, a conduit for goods in excess of $3 trillion every year.

As tension mounts over territorial claims in the area, the Philippine coast guard had early this year spotted a Chinese navy ship and dozens of militia vessels around the island, one of nine features Manila occupies in the Spratly archipelago.

Inaugurated on Friday, the new three-storey facility is equipped with state-of-the art technology such as radar, automatic identification, satellite communication, and coastal cameras, the Philippine coast guard said in a statement.

"The behaviour of the Chinese coast guard, People's Liberation Army (PLA) Navy and Chinese militias are sometimes unpredictable," Eduardo Ano, the Philippine national security adviser, said during a visit to the island.

"They do not adhere to the international order, to the rule of law," he told reporters on Friday.

"What they're describing as gray-zone tactics ... are pure bullying and it's purely illegal. It's not acceptable in the international order."

The Chinese embassy in Manila did not immediately respond to a request for comment.

Manila's outpost of Thitu is its biggest and most strategically important in the South China Sea, largely claimed by Beijing, despite conflicting territorial claims by several regional nations.

Known locally as Pag-asa, Thitu lies about 300 miles (480 km) west of the Philippine province of Palawan. Home to about 200 people, it is used by Manila to maintain its territorial claim.

Besides the Philippines, Brunei, China, Malaysia, Taiwan and Vietnam have competing claims of sovereignty in the South China Sea, a conduit for goods in excess of $3 trillion every year.