Taiwan has for the first time reported mainland Chinese law enforcement vessels near an island it controls in the South China Sea, a development experts say could form the basis of Beijing’s effective control in the waters.

The move comes after Beijing accused Taiwan’s ruling Democratic Progressive Party of inaction in the face of Japan-Philippines maritime delineation talks, which could concern exclusive economic zones claimed by Beijing and Taipei.

Two mainland Chinese law enforcement ships – the Sansha Zhifa 301 and Sansha No 2, which are both managed by the southern city of Sansha in Hainan province – entered the Taipei-claimed 3.2-nautical mile prohibited waters around Taiping Island at 8.28am on Thursday, Taiwan’s Coast Guard Administration (CGA) said. The vessels reportedly moved closer to the Taipei-defined 2.1-nautical-mile restrictive waters at 8.31am.

Taiwan said it was the first time mainland Chinese government vessels had entered the Taipei-defined restricted waters near Taiping – also known as Itu Aba – and that both vessels were driven away by its coastguard at 8.43am.

It added that a mainland Chinese coastguard ship had previously entered the prohibited waters of the Pratas Islands, also known as the Dongsha Islands, on June 5.

Experts said Beijing’s movement around the islands was an attempt to assert sovereignty through concrete action, which could form the basis for effective control in future.