Vietnam has demanded the unconditional release of nine fishermen detained by China last month, Vietnamese media reported today, as tension rose in the latest dispute in the South China Sea.

The demand came a week before a meeting in the Vietnamese capital, Hanoi, that will bring together defence ministers of the region as well as the US defence secretary, Robert Gates.

China has accused the fishermen, who were taken near the disputed Paracel Islands, of breaking the law by fishing with explosives and said they would be released after a fine is paid. Vietnam has responded by describing the arrest as "irrational".

"Fishing boat QNg 66478TS was carrying out its ordinary operations in Vietnam's Hoang Sa [Paracel] archipelago sovereign waters," a Vietnamese official was quoted as saying on the Vietnamnet website. "There were no explosives on the boat. On 15 September, Vietnam's embassy in China received a note from China which did not mention that the boat was carrying explosives."

Vietnam says the fishermen were on a routine fishing trip in Vietnamese waters, using fishing nets and lamps. Its foreign ministry has sent a diplomatic note to the Chinese embassy in Hanoi, stressing that the arrest had seriously violated Vietnam's sovereignty.

Asian countries have become increasingly nervous at China's growing assertiveness in the potentially oil-rich South China Sea, where China, Vietnam, Malaysia, the Philippines, Brunei and Taiwan all have competing claims. The region closely followed a dispute between China and Japan, when Japan detained a Chinese crew in early September for allegedly ramming Japanese patrol boats near disputed islands in the East China Sea. Japan released the crew and then the captain after coming under heavy pressure and threats from China.

The Vietnam-China spat could be an irritant at next week's high-level meeting in Hanoi of defence ministers from the Association of South East Asian Nations and eight other countries, including China, Japan and the US.

However, Gates will meet his Chinese counterpart, General Liang Guanglie, as the US and China move to end an eight-month freeze on military exchanges. China suspended military contacts in January in protest at US arms sales to Taiwan. China pointedly refused to invite Gates to Beijing during his trip to the region in June.

American officials have been keen to resume military contacts with China at a time of heightening tension in the Pacific. Japan, South Korea, Vietnam, Indonesia and Australia, have all moved to boost co-operation with the US in response to what they see as China's growing willingness to throw its weight around. China, in turn, sees such moves as US encroachment into its neighbourhood.

The US secretary of state, Hillary Clinton, angered China when she said recently that America had a "national interest" in seeing the territorial disputes resolved through a "collaborative diplomatic process by all claimants". China claims sovereignty over the entire 3.5m square-kilometre South China Sea, dotted with disputed groups of islands.