China may denounce them as "dangerous and provocative", but the United States has every intention of continuing to send its ships inside the 12 nautical miles from Chinese-built islands in the South China Sea.

"We are acting on the basis that we will fly, sail and operate wherever international law permits," US Defence Secretary Ashton Carter told the Senate Armed Services Committee in Washington on Tuesday.

Washington has long made it clear that it will not accept Beijing's strategy of transforming underwater reefs into islands in order to expand China's territorial waters.

International law favours the US position. China's claim to new 12-mile exclusion zones runs against legal conventions, which define islands as naturally formed territories that remain above water even when sea tides are high. Man-made islands cannot be used to establish territorial claims, as Beijing is now attempting.

This is not some obscure legal debate but a showdown about the freedom of navigation on the high seas, one that the US is determined to uphold. Still, it appears anxious to avoid inflaming matters, including giving Beijing plenty of advance warning about what it was proposing to do.

China has room to de-escalate the confrontation. There is no US demand to break up the artificial features, but merely to tacitly accept that these cannot be used to advance territorial claims. And China has an added incentive to compromise, since the US position that man-made islands cannot create maritime exclusion zones is applicable too to rival claimants Vietnam and the Philippines.

However, if China doesn't blink, the US won't either because to drop its navy patrols will be seen as an acceptance of Beijing's position. Both sides are locked in a confrontation from which they can get out; they need only the goodwill to do so.