While most of the mainland's international trade disputes have involved it being accused of flooding the rest of the world with cheap exports, the latest complaint ironically claims the country is preventing some commodities from entering the overseas market. The complaint to the World Trade Organisation filed jointly by the United States and the European Union accuses the mainland of restricting the amount of nine raw materials that can be made available to overseas buyers. Toeing the official line, mainland analysts and officials defended the moves as being conservation efforts. But observers were keener to highlight the apparent resurgence in US-China trade frictions after a period of calm since the US presidential election late last year. They were swift to point out that it was the first time the US had taken a dispute to the WTO since President Barack Obama was sworn in in January. 'Corporate America has recently grown quite aggressive [towards Chinese enterprises],' said Yang Chen, a lawyer with Beijing-based firm JT&N. Mr Yang's firm represented one of the firms targeted by a US investigation launched last month into alleged dumping by mainland steelmakers of welded and seamless steel pipes, mainly used for oil pipelines. Mainland enterprises reportedly sold a total of US$2.6 billion worth of the pipes in the US market last year, almost three times the total in 2007. The US Department of Commerce launched two similar probes earlier this month into complaints of Chinese companies flooding the US market with cheap steel grating, a product widely used in various industries from chemicals to food processing, and steel wires strands used in the construction industry. 'The intensity of the actions in the past two months contrasts starkly with a relatively quiet period between last October and April,' said Mr Yang. 'It reflects the economic and political situation in the US.' The fact that the most recent complaints were related to the steel industry highlighted deepening problems in the global steel market, analysts said. 'Usually, there is no right or wrong in international trade disputes,' said a researcher with the Ministry of Commerce. 'When a market collapses, the recriminations over unfair practices of others begin. It's as simple as that. The steel industry, [the victim of] a battered global property and construction industry, is a typical example.' So it is no surprise that coke, an essential material used in making steel tubes, topped the list of disputed items in the latest WTO complaint. In 2008, China produced 336 million tonnes of coke, according to US Trade Representative Ron Kirk. By putting export restraints on the raw material, the amount of exports dropped to 12 million tonnes, he said in Washington on Tuesday. But Xu Zhongbo, the chairman of steel industry consultancy Beijing Meitak, said the central government put a quota on coke exports for environmental protection reasons. Coke is derived from coal in a process that results in heavy air pollution. 'Beijing's logic goes that by imposing restraints on exports, it can subdue the urge of domestic firms to increase capacity and so reduce pollution,' said Mr Xu. Other products involved in the latest complaint - bauxite, fluorspar, magnesium, manganese, silicon metal, silicon carbide, zinc and yellow phosphorus - were often overly mined on the mainland, said Liu Zhongyuan, a commodity expert at Xiangcai Qinian Futures.