China has finished dredging sand and coral on a reef in the South China Sea, and is now laying concrete for a 3km runway.

High-resolution photos taken by satellite mapping firm Digital Globe on July 18 showed only two dredgers on Subi reef, part of the Spratly archipelago in the southern half of the South China Sea. But there were 54 large supply ships at Subi, up from 38 on June 5. There were also more concrete transport trucks on the island that China has created so far on the reef: 34, up from 20.

These suggest a ramp-up of construction and preparations for an airstrip, said geopolitical analyst and author Victor Robert Lee, a South China Sea analyst.

He said a runway with a parallel taxiway could be built on a straight segment 3km long and 250m wide.

A runway on that much land will be comparable to the one the United States has in Guam, which can accommodate a B-52 bomber.

Subi is 30 minutes by boat from Philippine-occupied Thitu island, 480km west of Palawan province.

Security officials warn that once Subi is operational as an air base and harbour, supply runs to Thitu - where there are about 300 civilian and military personnel - will become difficult. The officials say the Chinese could use Subi as a staging area to chase away planes or ships bringing supplies to Thitu.

Subi is one of seven reefs in the Spratlys where China has dredged tonnes of sand and coral to create islands big enough for runways, deep-water harbours and mall-size buildings. These man-made islands have sparked concerns that China is trying to control a waterway through which US$5 trillion (S$6.6 trillion) in global ship-borne trade passes every year.

China claims nearly all the 3.5 million sq km South China Sea, parts of which are contested by the Philippines, Vietnam, Malaysia, Brunei and Taiwan.

Meanwhile, in a move that may irk Vietnam, China is planning to start a second cruise ship link to the disputed Paracel islands in the northern half of the South China Sea, Reuters reported. Cruises began on a trial basis in 2013.