A group of US solar panel makers asked the Commerce Department on Thursday to consider imposing duties retroactively on Vietnam and Thailand due to a surge in imports, as those countries face probes for alleged unfair practices in the multibillion-dollar trade.

In May, the Commerce Department started investigations over silicon solar cells and panels made in Vietnam, Thailand, Malaysia and Cambodia. A group of domestic manufacturers alleges the products were sold in the US at excessively low prices and benefited from subsidies from China, home to many manufacturers with factories in the region.

The four Southeast Asian countries accounted for nearly 80% of US imports last year in dollar terms, according to US trade data reviewed by Reuters.

US President Joe Biden has pledged to revitalise American manufacturing by providing incentives for domestic production of goods to help fight climate change, including solar panels and electric-vehicle batteries that are mainly made in China. Some in the small US solar-manufacturing sector say the industry is struggling to compete with low-priced imports.

As speculation about the trade probes began circulating this year, exports from Vietnam and Thailand surged, the American Alliance for Solar Manufacturing Trade Committee said in a complaint filed with Commerce, which followed its earlier petition in April to start the trade investigations. The group represents domestic producers including Hanwha Qcells and First Solar.

That investigation could lead to high tariffs from as early as June, if US federal officials confirm unfair trading practices in preliminary determinations scheduled in early October, and uphold retroactive duties applicable 90 days before their decisions.

The trade ministries of Vietnam and Thailand did not reply to requests for comment.

The new tariffs could be particularly harmful to Vietnam, which risks the highest duties as it is deemed by the US as a non-market economy. That status usually leads to harsher sanctions because of unpredictable domestic pricing, according to trade experts.

Vietnam's estimated gap between domestic and export prices, known as dumping margins, were estimated by the US at over 270% using Indonesia as the benchmark, more than three times higher than Thailand's. Larger margins are likely to result in higher tariffs, if approved, experts said.

In their latest complaint, the US manufacturers said the volume of solar imports from Vietnam rose 39%, and that from Thailand increased 17%, in the second quarter compared to the first quarter, as the two countries allegedly increased shipments to the US ahead of potential duties.

Such moves could be considered "critical circumstances", US producers said. Both the Commerce Department and the International Trade Commission must find that critical circumstances exist for duties to be imposed retroactively.

  - Reuters