Vietnam and the Philippines on Tuesday lashed out at China's moves to establish a military garrison in the South China Sea, amid escalating tensions in the disputed waters.
Hanoi filed a formal protest with Beijing against the plan outlined by China this week to station troops in Sansha in the disputed Paracel Islands, saying it "violates international law".
Manila, which is involved in a dispute over another archipelago, the Spratly Islands, also weighed into the row, summoning the Chinese ambassador to lodge a complaint against the garrison announcement.
An intensifying spat over the South China Sea -- the site of key shipping routes and thought to have vast oil and gas reserves -- has seen a barrage of diplomatic moves between the countries with competing territorial claims.
Taiwan, one of several claimants to portions of the Spratly chain, plans to boost firepower at its base on that archipelago's biggest island Taiping from next month, Taipei's coastguard said on Tuesday.
Longer-range artillery and mortars are to be added to existing weaponry at the site, in a move that could further stoke tensions in the region.
China says it owns much of the South China Sea, while Vietnam, the Philippines, Taiwan, Brunei and Malaysia each claim portions.
The disputes have become particularly acrimonious in recent weeks, with Vietnam and the Philippines criticising what they call Chinese encroachment.
Beijing's garrison plan "violates international law, seriously violates Vietnam's sovereignty... and is invalid," Foreign Ministry spokesman Luong Thanh Nghi told AFP.
China attracted Hanoi's ire -- and sparked a series of rare protests in the Vietnamese capital -- when it last month designated Sansha as its administrative centre for the Paracels and the Spratly Islands.
The state-backed China National Offshore Oil Corporation also announced it was welcoming bids to explore oil blocks in the disputed waters, a week after Vietnam adopted a law placing the Spratlys under its sovereignty.
Nghi told AFP Tuesday that China must revoke its "wrongdoings" and urged "friendly and cooperative" relations in order to "maintain peace and stability" in the South China Sea.
China and South Vietnam once administered different parts of the Paracels but after a brief conflict in 1974 Beijing took control of the entire group of islands. Vietnam still holds several of the larger Spratlys.
A July 13 meeting of the Association of Southeast Nations broke up without a joint statement for the first time in 45 years because members could not agree on how to refer to China's behaviour in the disputed waters.
The countries are drafting a "code of conduct" to try to prevent flare-ups in the area.