WASHINGTON ?- The United States on Tuesday expanded its challenge at the World Trade Organization (WTO) to China's export restraints on key raw materials.

It requested consultation with China on the country's export duties on chromium, as well as China's export quotas on antimony, indium, magnesia, talc and tin, said the Office of the US Trade Representative (USTR).

USTR claimed that China's export restraints on these materials, including duties and quotas, provide an unfair competitive advantage to China at the expense of American workers and manufacturers.

On Tuesday, the European Union also launched a legal challenge at the WTO to China's duty and quota requirements on the export of 11 raw materials, including graphite, cobalt, copper, lead, chromium, magnesium, talcum, tantalum, tin, antimony and indium.

China's Ministry of Commerce (MOC) said in a statement Tuesday that the duty and quota requirements on the export of 11 raw materials conform to WTO rules,

The export controls on graphite, cobalt, copper, lead, chromium, magnesium, talcum, tantalum, tin, antimony and indium are imposed to protect the country's resources and the environment, said the statement.

The MOC said China will properly handle the case in accordance with WTO dispute settlement procedures.

The US challenges over China's export restraints on raw materials came at a time of increasing anti-trade rhetoric in the current US presidential campaign.

The Obama administration wants to demonstrate a tough stance on enforcing trade agreements, in a bid to draw lawmakers support to ratify the Trans-Pacific Partnership trade agreement, a top legislative priority for President Obama this year.

Last week, the United States also requested consultation with China at the WTO over its export duties on nine different raw materials including antimony, cobalt, copper, graphite, lead, magnesia, talc, tantalum and tin.

Consultations are the first step in the WTO dispute settlement process. If the United States and China are not able to reach a mutually agreed solution through consultations, the United States may request that the WTO establish a dispute settlement panel to examine the matter.