Chinese manufacturers have been illegally selling steel towers for wind turbines below the cost of production and will have to pay duties of 20.85 percent to 72.69 percent on imports, the US Commerce Department said Friday in a preliminary ruling in an antidumping case brought by four US tower makers.

The department said it found similar dumping on the part of Vietnamese manufacturers and set duties at 52.67 percent for CS Wind and 59.91 percent for all other Vietnamese companies.

The finding is the fourth this year in favor of US wind and solar manufacturers and is likely to intensify tension with the Chinese, who have been rapidly expanding manufacturing capacity for alternative energy technologies and selling to global markets with inexpensive products, especially solar panels.

Earlier this year, the Commerce Department ruled that China was dumping solar panels on the US market and imposed duties of 31 percent on most of the imports, which added to earlier duties imposed over what the department said were unfair subsidies for its manufacturers.

The Chinese government has responded to the complaints by beginning its own investigation into whether US and Korean manufacturers of polysilicon, the main ingredient in the solar panels, were selling the material below cost.