Image caption China and the US are involved in trade disputes involving a range of products including steel

The World Trade Organisation (WTO) has upheld its decision that China's tariffs on imports of certain US steel products were illegal.

Beijing had imposed duties on a particular kind of US steel, alleging that its makers were being given subsidies by the US government.

The WTO ruled against the tariffs in June, a decision it upheld saying that China had failed to prove its charges.

The case is the latest in a series of trade conflicts between the countries.

"Today we are again plainly stating that we will continue to take every step necessary to ensure that China plays by the rules and does not unfairly restrict exports of US products," US trade representative Ron Kirk said.

'Beating China'

Disagreements between China and the US have been growing in recent times and the two have sparred over issues ranging from China's currency policies to allegations of state subsidies given to Chinese firms.

The US has upped its ante against Beijing recently, not least because of the upcoming presidential elections.

The impact of China's growing economic might on the US economy, and how the US should respond to it, has become a key issue in the elections.

[It's] a small benefit for the Obama campaign because it can advertise 'beating China' in Ohio Derek Scissors, Heritage Foundation

Both President Barack Obama and challenger Mitt Romney have pledged to put more pressure on Beijing.

If elected, Mr Romney has vowed to formally label China a "currency manipulator" on his first day in office, opening the way to trade sanctions.

Last month, Mr Obama filed a complaint against China with the WTO, accusing it of illegally subsidising car exports.

That was in addition to two other complaints the Obama administration has filed with the WTO against China this year.

The first accused Beijing of restricting its exports of so-called rare earth metals, used to make high-tech devices.

The second accused Beijing of unfairly imposing anti-dumping duties on US cars exported to China, making them more expensive to buy. The US alleged that the move was designed to protect Chinese manufacturers.

But despite these actions, Mr Romney has accused Mr Obama of being too soft on China.

Analysts said the latest decision by the WTO was likely to provide a boost to Mr Obama's campaign.

"[It's] a small benefit for the Obama campaign because it can advertise 'beating China' in Ohio," said Derek Scissors, a research fellow at the Heritage Foundation.