WASHINGTON: The United States pushed ahead on Friday with two new investigations into charges of unfair trade practices by China, but rejected a third case one week ahead of President Barack Obama's trip to Asia.

US lawmakers also asked Obama to renew his support for legislation targeting China's exchange rate practices.

The US International Trade Commission approved probes into imports of glossy magazine-quality paper from both China and Indonesia totaling hundreds of millions of dollars, as well as certain salts from China that are used in cleaning products, food additives and fertilizer.

The steel pipe case is biggest US trade action against China to date and Beijing quickly denounced the new duties.

It also launched its own investigation into imports of American automobiles and sports utility vehicles, which it says have benefited from a long list of incentives and tax breaks granted by the US government and the state of Michigan.

Steve Collins, president of the American Automotive Policy Council, said they had just received the documents containing the charges and were still reviewing the details of the case.

Meanwhile, a bipartisan group of 45 lawmakers in the House of Representatives urged Obama in a letter to declare support for their bill allowing the Commerce Department to slap duties on imports from countries with "misaligned currencies."

Obama backed similar legislation when he was a member of the Senate and running for president.

The bill is aimed mainly at China, which lawmakers accused of deliberately undervaluing its currency against the dollar to give its companies an unfair trade advantage.

They blame China's exchange rate practices for a big chunk of the US trade deficit with China, which totaled $143.7 billion in first eight months of 2009.

INVESTIGATIONS RISE

Antidumping and countervailing duty investigations often rise during times of economic stress.

Since taking office in January, the Obama administration has initiated at least a dozen such probes against China in response to petitions filed by US companies or unions. ?

US law requires the US International Trade Commission, an independent trade body, to approve any investigation launched by the Commerce Department.

In a relatively rare example of that, the ITC on Friday rejected an inquiry into whether Chinese mainland and Taiwan were subsidizing and dumping certain steel fasteners in the United States. It said there was not enough evidence US manufacturer Nucor Fastener had been harmed or threatened with harm by the imports.

Of the two cases approved by the ITC on Friday, the one involving glossy paper revives a case brought by US manufacturers over two years ago.

In the earlier investigation, the Commerce Department set duties on imports from China, Indonesia and South Korea but those did not survive a final ITC review and decision.

The ITC's vote on Friday to approve a new narrower probe was a victory for the United Steelworkers union, a driving force behind the steel pipe case and Obama's decision in September to slap a 35-percent duty on Chinese-made tires.

"We cannot stand by and let imports unfairly take away our good jobs and shutdown factories," Leo Gerard, president of the steelworkers union said in a statement.

But US paper importer, Global Paper Solutions, blasted the ruling, which it said was unwarranted and sent a protectionist message ahead of Obama's 10-day trip to Japan, Singapore, China and South Korea beginning next week.

The United States imported $229 million of the glossy paper covered by the investigation from China in 2008 and $53 million from Indonesia.

US producers NewPage Corp, Appleton Coated LLC and Warren Company joined with the steelworkers union in asking for anti-dumping duties ranging up to 136 percent on imports from China and up to 41 percent from Indonesia.

The companies also want additional "countervailing" duties on the imports to offset alleged Chinese government subsidies.

In the salts case, US companies ICL Performance Products and Prayon Inc asked for antidumping duties ranging up to 178 percent and additional countervailing duties on some $31.5 million of imports from China.

The ITC said there was sufficient evidence to launch an investigation into imports of three specific salt products -- tetrapotassium pyrophosphate, monopotassium phosphate, and dipotassium phosphate.

However, it excluded a fourth salt product, sodium tripolyphosphate, from the probe.