A rapid succession of incidents in the South China and East China seas has the US worried.

A rapid succession of incidents in the South China and East China seas has the US worried.

In 1947, the then nationalist government of China published a map of the South China Sea, marking with dashes a vast sweep of water close to the coastlines of Vietnam, Borneo and the Philippines as historical Chinese territory.

The regime in China soon changed, but the claim and the enclosed area have remained much the same, albeit with a varying number of dashes (the ''nine-dash line'' being the most common reference). It's brought Beijing and the nationalist remnant in Taiwan into diplomatic disputes and periodic conflict with south-east Asian claimants of islands, reefs and resource zones.

Now a rapid succession of incidents in the South China and East China seas are creating concern in the US that there is a plan behind them.

As well as attempted intimidation of allied and friendly smaller nations over territory and resources, could they also represent an ''incremental effort'' by China to impose control over ship and air movements in international sea and airspace?

In the south, the incidents have included blocking access for Filipino fishermen on the disputed Scarborough Reef, harassing a Philippines outpost on another shoal, opening bids for hydrocarbon exploration blocks close to the shores of other claimants, setting up administrative and military commands covering disputed islands, and updating fishing regulations that require foreign boats to obtain licences.

In the east, China's maritime surveillance agencies have sent ships and aircraft around the Japanese-controlled Senkaku islands, which in Chinese eyes were illegally annexed nearly 120 years ago. In November, Beijing announced an Air Defence Identification Zone (ADIZ) covering the disputed islands, requiring aircraft crossing the zone to notify Chinese authorities or risk ''emergency defensive measures''.

The risk for the US lies in clashes that might draw it to help an ally. Japan has a strong defence force; the Philippines has virtually no naval or air combat capability, but is starting to see a direct challenge to its own national interest.

The ADIZ declaration (and talk of one being prepared for the South China Sea), the jostling of a US hydrographic ship off Hainan four years ago, and the USS Cowpens incident in December (when a Chinese warship cut across the bows of the American cruiser in the South China Sea) suggest small steps to convert both seas into zones where China regulates who enters and what they can do, which is anathema to a maritime superpower.

This week, Washington sent what diplomats regard as an unusual message to Beijing, in a public statement declaring the ''nine-dash line'' was ''fundamentally flawed'' under international law, which bases maritime rights on land features, coastlines and islands.

The push-back came in testimony on Tuesday to the Asia subcommittee of the House of Representatives foreign affairs committee by Daniel Russel, a mild-appearing career diplomat who became assistant secretary in the Department of State for East Asia and the Pacific in December.

His words went over the heads of the committee members - one chided him for ''diplomatic mumbo-jumbo'' - but were unusually sharp: ''There is growing concern that this pattern of behaviour in the South China Sea reflects an incremental effort by China to assert control over the area contained in the so-called 'nine-dash line', despite the objections of its neighbours and despite the lack of any explanation or apparent basis under international law regarding the scope of the claim itself.''

The official suggested China clarify or modify the nine-dash-line claim to bring it in alignment with the International Law of the Sea, thereby making resolution of conflicting claims possible in a more peaceful, non-coercive way. Meanwhile, all claimants should refrain from attempts to change the status quo.

Russel's statement is seen as an effort to firm the US's position, after what Japan and others saw as an ambivalent response to the East China Sea ADIZ declaration (the US military sent B-52 bombers through the zone, undeclared to China, while the US civil aviation agency advised airlines to notify flight paths).

''It was calling out China on the nine-dash line,'' one senior diplomat said.

♦ Hamish McDonald, a senior journalist with the Australian National University's College of Asia and the Pacific, is in Washington at the Woodrow Wilson Centre as a public policy scholar.