Vietnam has accused China again of harassing a ship conducting seismic surveys in the South China Sea. This is the second such incident in two weeks to increase tensions between the neighbouring countries with competing maritime claims. Chinese Foreign Ministry spokeswoman Nguyen Phuong Nga said its fishing boat deployed a cable cutting device and got it trapped in a network of underwater cables in use by a ship hired by Vietnam. The ship was operating over Vietnam's continental shelf and within its exclusive economic zone off the southern coast, she said. Two Chinese ships then came to help the Chinese fishing vessel, she said, calling the incident part of a campaign of systematic and intentional violations by China. Vietnam lodged a complaint with China in late May when a Chinese patrol vessel slashed the cables of a Vietnamese ship conducting a seismic survey off its south-central coast. The two countries, which fought a brief war on their land border in 1979, have since exchanged accusations and re-staked long-standing claims of sovereignty over maritime territory in the South China Sea which covers important shipping routes and may hold large oil and gas reserves. On Sunday, up to 300 people gathered in Hanoi and several thousand marched in Ho Chi Minh City in a rare public protest against China's assertiveness over its maritime claims. The Philippines, Malaysia, Brunei and Taiwan also claim territories in the South China Sea, but China's claim is by far the largest, forming a vast U-shape over most of the sea's 1.7 million sq km.