Offers of birdwatching, holiday cruises and flying tours have rekindled tensions over disputed atolls and reefs in the South China Sea. In March, Taiwan built a birdwatching hide on one of the specks of territory it claims in the Spratly Islands. Last month, Vietnam launched what it said was the first of a series of holiday cruises to cays it occupies in the Spratly chain and began work to restore a disused airstrip on one. Vietnam protested at Taiwan's activity, while Beijing protested at Vietnam's moves, taking the opportunity to reiterate its assertion of 'indisputable' sovereignty over the Spratlys, which it calls the Nansha Islands. The People's Liberation Army held military exercises in the South China Sea last month, sending a signal to rivals to back off. Malaysia, the Philippines and Brunei - the other claimants to parts of the widely scattered Spratlys - were alarmed, not least because it was in blatant defiance of the spirit of a non-binding joint declaration signed in November 2002 by mainland China and the 10 members of the Association of Southeast Asian Nations. It said that disputes should be resolved peacefully, and called on claimants to avoid action that might cause tension. Military clashes and skirmishes in the South China Sea have occurred numerous times over the past few decades. The most serious were in 1974, when mainland forces invaded and captured the Paracel Islands (north of the Spratlys) from Vietnam, and in 1988, when the PLA and Vietnamese navies clashed near a reef in the Spratlys. Several Vietnamese vessels were sunk and more than 70 Vietnamese sailors killed. Why is there a resurgence of tension now, after all the diplomatic efforts? It can hardly be about ornithology and tourism. A significant part of the answer is the increasing urgency for major East Asian economies, led by China and Japan, to secure disputed maritime frontiers and gain control over resources, especially offshore oil and gas, seen as vital for future growth. In March, seven mainland activists, apparently with the connivance of the government in Beijing, landed on the Diaoyu Islands in the East China Sea between southern Japan and Taiwan. The cluster of rocky outcrops, which the Japanese call the Senkaku Islands, are claimed by Japan, mainland China and Taiwan. Although uninhabited, they are patrolled by Japan's coastguard. The activists were rounded up and deported. Geological and seismic surveys around the Diaoyus have indicated that there are sizeable oil and natural gas reserves. The South China Sea has proven oil reserves estimated at about seven billion barrels, while oil production in the region is currently about 2.5 million barrels per day. Gas reserves and output are larger still. Maps published by China mark its maritime boundary as a broken line extending deep into the South China Sea. Its claim appears to encompass not just the Paracel and Spratly islands, but many of the offshore oil and gas fields being exploited or explored by Southeast Asian countries close to their territory. China evidently sees the Spratlys and Diaoyus as potentially rich petroleum provinces that are all the more important because they could ease dependence on increasing energy imports from more distant sources, including the volatile Persian Gulf. Under international law, islands can be the key to ownership of huge swathes of the seabed and continental shelf that surrounds them. So whoever is in undisputed or effective control of the seemingly insignificant specks may be able to exploit any oil and gas there. Michael Richardson, a former Asia editor of the International Herald Tribune, is a visiting senior research fellow at the Institute of South East Asian Studies in Singapore. The views expressed in this article are those of the author