The US has started a new series of games with China by sending its guided missile destroyer USS Lassen within 12 nautical miles of China's isles in the South China Sea.

We need to define the legal status of built-up islands and reefs according to the 1982 UN Convention on the Law of Sea. It should be made clear that the land claim projects are carried out in China's land and islets, rather than as the US media claim being artificial islands that have no territorial sea of their own.

According to Article 121 of UNCLOS, the territorial sea, the contiguous zone, the exclusive economic zone and the continental shelf of an island are determined in accordance with the provisions of this Convention applicable to other land territory.

Article 6 of UNCLOS says: "In the case of islands situated on atolls or of islands having fringing reefs, the baseline for measuring the breadth of the territorial sea is the seaward low-water line of the reef..."

Since people can't habitat reefs, exclusive economic zones can't be drawn from them.

However, according to Article 13: "A low-tide elevation is a naturally formed area of land which is surrounded by and above water at low tide but submerged at high tide. Where a low-tide elevation is situated wholly or partly at a distance not exceeding the breadth of the territorial sea from the mainland or an island, the low-water line on that elevation may be used as the baseline for measuring the breadth of the territorial sea."

Some of China's land reclamation projects in South China Sea are carried out on reefs and low-tide elevations. But after the completion of the land reclamation projects it is impossible to prove that the islets used to be low-tide elevations that can't claim territorial sea.

And according to Article 17: "Subject to this Convention, ships of all States, whether coastal or land-locked, enjoy the right of innocent passage through the territorial sea."

Therefore, according to UNCLOS, the US can enter the adjacent waters of such islands and reefs in the name "innocent passage", even the waters are China's territorial sea. But the US congress hasn't ratified UNCLOS yet, which makes things difficult. What's more, there is a heated debate over whether warships enjoy the "innocent passage" within other countries' territorial seas. And it should be noted that "innocent passage" goes both ways and therefore the Chinese navy are entitled to the same right to enter the waters adjacent to US islands and reefs according to the US' logic.

As for the reefs standing above high tide before construction, China owns the right to the territorial seas. If the US wants innocent passage through these waters, it should apply for China's permission and be in accordance with China's Law on the Territorial Sea and the Contiguous Zone.

The US' bold military provocation will not only damage Sino-US relations, but also could result in the risk of conflicts in the waters if it persists.

It is obvious that US took innocent passage as the excuse for its warship USS Lassen entering into or getting close to China's territorial seas. This is similar to the US' selling weapons to Taiwan even after China and US established their diplomatic relationship. There is no international law making it legal to sell weapons to parts of a sovereign country.

The US' true reason for sailing its warship was because it fears China's growing capability to project its power after the completion of its land reclamation projects in the South China Sea.

Since China can enhance management of larger area of waters, this may further change the geopolitical pattern, and the US worries that the change will undermine its dominant position in Asia-Pacific.

Building up islands or reefs, however, is allowed under international law. The US can't slow China's pace of building up its islands and reefs by simply flexing its military muscle in the South China Sea.

The US should face up to the reality that it's no longer a unipolar world, and seek to engage more with rather than contain China.

The author is a professor at and associate dean of the Institute of International Studies in Fudan University, Shanghai.