WASHINGTON — Chinese manufacturers of towers for wind turbines received unfair subsidies and must now pay duties of 13.7 to 26 percent, the Commerce Department said on Wednesday in a preliminary decision in a case brought by four American manufacturers of the towers.

The decision, the third trade case decided this year in favor of American wind and solar manufacturers, will be followed by another in the coming weeks on whether Chinese companies engaged in dumping the towers in the United States at prices below the cost of making them.

This month, the Commerce Department said China was dumping solar panels in the American market and imposed duties of 31 percent on the imports, adding to earlier duties imposed under a department ruling that China unfairly subsidized its solar manufacturers.

Some trade experts have warned that the series of rulings will increase trade tensions with China, which has already filed its own complaints about subsidies that federal and state governments give to American wind and solar companies.