Vietnam on Wednesday protested to China over what it said was an attack on a Vietnamese fishing boat three days ago in contested South China Sea waters that injured several fishermen.

The Vietnamese foreign ministry said in a statement that Chinese law enforcers beat the Vietnamese fishermen and took away their fishing equipment when their boat was operating near Hoang Sa, Vietnam’s name for the Paracel Islands.

The Chinese-controlled islands, also claimed by Vietnam, are in the South China Sea, a busy global maritime waterway, almost all of which is claimed by China.

“Vietnam is extremely concerned, indignant and resolutely protests the brutal treatment by Chinese law enforcement forces of Vietnamese fishermen and fishing boats operating in the Hoang Sa archipelago of Vietnam,” foreign ministry spokesperson Pham Thu Hang said in a statement.

The ministry delivered a strong protest to the Chinese embassy in Hanoi demanding that China respect Vietnam’s sovereignty, investigate the incident and desist from further such actions, Hang said.

Vietnamese state media reported this week that around 40 people from two foreign vessels had beaten the fishermen with iron pipes, injuring 10.

China’s foreign ministry said on Tuesday that the Vietnamese boats had been fishing illegally in Paracel waters without the permission of the Beijing government, and that Chinese authorities had taken measures to stop them.

“The on-site operations were professional and restrained, and no injuries were found,” it said in response to a Reuters request for comment, without specifically referring to the attack.