Nearly 300 armed Chinese police officers and soldiers took part in an emergency drill near the Xisha Islands (Paracel Islands) in the South China Sea.

Increasingly assertive action by China's coast guard ships in the South China Sea risks destabilizing the region, according to the authors of new research tracking maritime law enforcement incidents across the vital trade route.

While the risks of full-blown naval conflict dominates strategic fears over the disputed waterway, the danger of incidents involving coast guards should not be underestimated, said Bonnie Glaser, a regional security expert at Washington's Center for Strategic and International Studies think-tank.

CSIS researchers have detailed some 45 clashes and standoffs in the South China Sea since 2010, in a survey due to be published week on its ChinaPower website and seen by Reuters.

While the research includes clashes between a variety of regional states and types of vessels, the actions of China's coast guard dominates the picture. China's coast guard has been involved in 30 of the cases logged, two-thirds of the total. Four other incidents involved a Chinese naval vessel operating in a law enforcement capacity.

"The evidence is clear that there is a pattern of behavior from China that is contrary to what law enforcement usually involves," Glaser told Reuters.

"We're seeing bullying, harassment and ramming of vessels from countries whose coast guard and fishing vessels are much smaller, often to assert sovereignty throughout the South China Sea."