Vietnam condemned China on Thursday while saying that Chinese law enforcement personnel assaulted 10 Vietnamese fishermen, damaged their fishing gear and seized about 4 tons of fish catch near the disputed Paracel Islands in the South China Sea.

The fishermen first reported the assault near the Chinese-controlled islands by radio on Sunday but did not identify the attackers.

Three of the fishermen suffered broken limbs and the rest sustained other injuries, according to Vietnamese state media. Some were taken on stretchers to a hospital after they returned to Quang Ngai province late Monday.

Vietnam’s Ministry of Foreign Affairs blamed Chinese law enforcement personnel on Thursday for the high-seas attack, saying it had “seriously violated Vietnam’s sovereignty in the Paracel Islands,” international law and an agreement by the leaders of the rival claimant countries to better manage their territorial disputes.

Chinese officials did not immediately issue a reaction.

Vietnam conveyed its protest and alarm over the attack to the Chinese ambassador in the Vietnamese capital, Hanoi.

Vietnam demanded that Beijing respect its sovereignty in the Paracel Islands, launch an investigation and provide Hanoi with information about the attack, Vietnamese spokesperson Pham Thu Hang said in a statement posted on the Foreign Ministry’s website.

China has become increasingly aggressive in asserting its claims in virtually the entire South China Sea, through which about $5 trillion in global trade transits each year. The busy sea passage is also believed to be sitting atop vast undersea deposits of oil and gas.

Aside from China and the Philippines, Vietnam, Malaysia, Brunei and Taiwan also have overlapping claims in the strategic waterway.

The United States has no claims in the disputed waters, but it has deployed Navy ships and Air Force fighter jets to patrol the waterway and promote freedom of navigation and overflight. China has warned the U.S. not to meddle in what it says is a purely Asian dispute.

The Vietnamese newspaper Tien Phong cited one of the fishermen, Tran Tien Cong, as saying that two foreign boats approached them from the rear and that personnel from those vessels boarded their boat and started beating the fishermen with a meter-long (three-foot-long) stick, apparently made of iron.

The Vietnamese fishermen panicked and did not fight back because they were overwhelmed by an estimated 40 attackers. Another fisherman, Nguyen Thuong, was cited as saying that the attackers, who spoke through a translator, ordered them to sail back to Vietnam. The assailants then seized their fishing gear and fish catch.

After being beaten, the Vietnamese fishermen were forced to kneel and were covered with plastic sheets before the attackers left.

The Paracel Islands lie about 400 kilometers (250 miles) off Vietnam’s eastern coast and about the same distance from China’s southernmost province of Hainan. Both countries, along with the self-governing island of Taiwan, claim the islands.

The islands have been under the de facto control of China since 1974, when Beijing seized them from Vietnam in a brief but violent naval conflict.

Last year, satellite photos showed that China appeared to be building an airstrip on Triton Island in the Paracel group. At the time, it appeared the airstrip would be big enough to accommodate turboprop aircraft and drones but not fighter jets or bombers.

China has also had a small harbor and buildings on the island for years, along with a helipad and radar arrays.

China has refused to provide details of its island construction work other than to say it is aimed at promoting global navigation safety.

It has rejected accusations, including by the U.S., that it is militarizing the sea passage.