A CHINESE Coast Guard vessel blocked and pursued a Philippine National Mapping vessel BRP Hydrographer Ventura and the Philippine Coast Guard's BRP Gabriela Silang as they sailed for the Bajo de Masinloc, according to Manila's GMA News.

The Philippine vessels were on their way to the Bajo de Masinloc, also known as the Scarborough Shoal, to conduct a hydrographic survey.

However, the CCG vessel blocked the Philippine vessels 35 nautical miles from Luzon's coast - the limit of China's nine-dash line, but at a location within the Philippine exclusive economic zone.

The vessels were halted for about eight hours, but the Philippine vessels eventually set sail for the Bajo de Masinloc with the CCG following them.

"China is being especially aggressive and making the point that it believes in this 9-dash claim," said international advocacy group SeaLight director Ray Powell.

SeaLight was started by volunteers at Stanford University's Gordian Knot Centre for National Security Innovation, which seeks to solve difficult defence and security challenges.

The SeaLight director recounted that China had harassed Vietnam as the latter conducted a hydrographic survey in its EEZ, even allegedly attempting to cut the cables of Vietnamese equipment.

He thus suggested that the Philippines be on guard against any actions China would take against the hydrographic survey.

"If indeed China decides this hydrographic survey is a threat to their sovereign claims in the 9-dash line, they could and they have shown in the past that the passage could get pretty aggressive with these things," Mr Powell said.