Shipping between China and Taiwan is buffeted by constant political squalls, but another carrier is moving to take advantage of what's available.

Yangming Marine Transport Corp., Taiwan's second-largest shipping company, plans to transfer six container ships to a Liberian unit beginning next month. Direct cross-strait sailings are banned by Taiwan, but ships registered abroad are permitted to ply the route.The Yang Ming ships will operate on existing routes between Asia and the U.S. West Coast with a stop in Shanghai, a spokesman for the line said Monday.

He forecast additional revenue of more than $4 million a year arising from the mainland calls.

''The inclusion of (Shanghai) will bring more business and cut costs,'' the spokesman said.

China has opened just two ports to Taiwan business - Fuzhou and Xiamen in the southeast - resisting pleas by Taiwanese and other carriers to allow access to mainline gateways such as Shanghai and Dalian in the northeast.

Yang Ming evidently hopes to skirt this prohibition by adding Shanghai to routes that call at points other than Taiwan. China had no immediate comment on the strategy.

Analysts say China is refusing to open its main ports to indirect Taiwan sailings because of the restrictions the island imposes on that business. Mainland cargo must be for transshipment, ships must call somewhere else en route, and goods must be loaded and unloaded at a designated offshore zone at the Port of Kaohsiung.

Because the goods don't officially enter Taiwan's customs area, the government can maintain its stance of forbidding direct trade. China, which regards the island as a renegade province, wants those restrictions lifted.

Hong Kong's Orient Overseas Container Line Ltd. opened a service from northern China to both northern and southern Taiwan last month in a venture with Fairweather Steamship Co. of Taiwan.

To get around Taiwan's restrictions, the service calls at a Japanese port-of-convenience on the islet of Ishigaki.

Two ships with capacity equivalent to 900 20-foot containers launched the weekly, fixed-day run. A third vessel of similar size is expected to be added, OOCL said.

A mainland Chinese ship called at Taiwan's northern port of Keelung last month, also via Ishigaki. The Panama-flagged vessel is operated by a Hong Kong subsidiary of Shanghai's Jinjian Shipping Co.

That firm operates six ships under the control of the Shanghai municipal government, but using the Hong Kong company meets Taiwan's requirements even though the territory is now part of China.

The mainland's state-run China Ocean Shipping Co. is awaiting approval for service to Kaohsiung. Officials there say it must operate under a Hong Kong company as the Shanghai ship is doing.

Annual volume between the two sides is about 1.5 million TEUs, nearly all routed through Hong Kong. Eliminating that stop could save $100 million a year, industry executives say.

Separately, Yang Ming said it will expand its services in the Asia-Mediterranean trade in cooperation with its partners in the New World Alliance.