Vietnam said Thursday that a Chinese vessel rammed into and sank one of its fishing boats near disputed islands in the South China Sea in an incident that could further raise tensions between the countries.

The 10 fishermen on board were rescued by another Vietnamese fishing boat and the case was reported to authorities when the fishermen returned home two days ago, said Phan Huy Hoang, president of central Quang Ngai province's Fisheries Association.

"Chinese actions against fishermen from Quang Ngai province have been more aggressive and brutal," Hoang said by telephone.

More than 20 Vietnamese fishing boats have been attacked by Chinese vessels this year including another that sank in July, he said, adding a formal protest will be made to China.

Boat captain Dang Dung said he and the nine crew members were sleeping after a night of fishing near the Paracels islands on Sept. 29 when the Chinese vessel slammed the side of their boat and five Chinese men jumped aboard. Dung said by telephone from Quang Ngai that the men carried knives and took away his boat's navigation devices, fishing equipment and their catch.

Dung said his leaking boat sank about 12 hours later, and the crew then spent four hours floating with life vests before being rescued.

It was not clear if the Chinese vessel belonged to the government.

Chinese Foreign Ministry spokeswoman Hua Chunying said, "Chinese authorities have the rights to take law enforcement measures in accordance with the law on boats that have illegally entered" waters the country claims as its own territory.

Vietnam's Foreign Ministry said it was investigating the fishermen's allegations and reiterated that it strongly protested inhumane treatment of Vietnamese fishermen in any circumstances.

"If there is such a case that foreign law enforcement commits acts that hinder the normal and legitimate operations of Vietnamese fishermen in this area, we will have formal and appropriate response," ministry spokesman Le Hai Binh said.

Vietnam and China both claim the Paracel Islands, which were occupied by China after ousting the U.S.-backed South Vietnamese navy in 1974, one year before the end of the Vietnam War.

The two countries, along with the Philippines, Malaysia, Brunei and Taiwan, also claim parts or all the Spratlys islands in the South China Sea, an area which occupies a busy international sea lane and is rich in oil and gas and fish resources.