HONG KONG: Satellite images obtained by this news agency on Thursday show a build-up of Chinese civilian vessels near contested Thitu Island, Manila`s key outpost in the South China Sea, but a senior Philippine navy officer said they are `not a cause for concern`.

One of the images taken by Maxar Technologies shows about 60 vessels, some within 2 nautical miles of Thitu, a strategically important island from which Manila monitors Chinese vessels and aircraft in the busy waterway.

Vice Admiral Alfonso Torres, chief of the Philippines` Western Command, said it was common for `maritime militia` ships to gather in the area. Manila, the Pentagon and foreign diplomats say such vessels work with the Chinese coast guard and navy to strengthen Beijing`s presence in disputed waters.

Rear Admiral Roy Trinidad, Philippines Navy spokesman for the South China Sea, also said maritime militia ships were regularly in the area, adding that Manila was aware of the vessels, which he called an `illegal presence`, but there was no need for alarm.

`It`s not a cause for concern,` Trinidad said. `We don`t have to read every action and react to that... What is important for us is to maintain our posture.` Online ship trackers show that many of the vessels in the satellite photos are Chinese-registered fishing craft.

The island, which the Philippines calls Pag-Asa, is Manila`s biggest and most strategically important in the disputed South China Sea, which is largely claimed by China and through which billions of dollars worth of goods pass each year. A 2016 ruling by the Permanent Court of Arbitration in the Hague found that Beijing`s expansive claims had no basis under international law.

The build-up comes after months of clashes and rammings between Chinese coast guard and fishing vessels and Philippines ships, particularly at the Scarborough and Second Thomas Shoals.

Thitu is close to a Chinese naval base and runway on Subi reef, which has sometimes served as a port for large numbers of Chinese maritime militia vessels, Trinidad said.-Reuters