BEIJING, July 10 (AP) -- China will tax oil exported by the foreign partners in offshore oil exploration joint ventures from Aug. 1, the Ministry of Finance said Tuesday.

Contracts already signed and in use will be exempt from the new duty until Aug. 1, 2012, the ministry said in a joint statement with the General Administration of Customs. Refunds will be paid on any taxes already collected on oil exports pre-dating Aug. 1, the ministry said.

China has been trying to discourage exports of coal and oil to maximize domestic use of its energy resources.

The new export tax follows the imposition of a windfall tax on sales inside China imposed with little advance warning 16 months ago, prompting several foreign companies to complain.

The "Special Upstream Tax Levy" was applied to oil produce onshore and offshore China, and assessed at between 20 and 40 percent of the portion of the price over US$40 a barrel.