Hawks in the People's Liberation Army have called for sanctions on American companies after Washington cleared a sale of advanced Patriot air defence missiles to Taiwan. Beijing immediately hit back at the arms sale with its standard rhetoric. Foreign Ministry spokeswoman Jiang Yu said yesterday the mainland had lodged a formal complaint with the United States. 'We urge the United States to recognise the serious harm that selling weapons to Taiwan would bring [to our relationship],' she said. Jiang said the US should stop selling Taiwan weapons in order to avoid jeopardising the overall co-operation framework between the two countries. But state-run China News Service yesterday ran comments by military academic, Rear Admiral Yang Yi , whose opinion was more aggressive than the foreign ministry. He called on the government to impose sanctions on American companies and stop 'forbearing' issues that would hamper Sino-US relations. 'Why don't we strike back at them as defence? Apart from protesting and adopting necessary measures, why not impose sanctions on those troublemakers,' Yang said. 'We should let enterprises and interest groups that are undermining Chinese people's interests suffer from disastrous damages. We should let them bear economic damages that are greater than the profit they have gained from the arms sale.' Such fierce comments are rarely made in public, especially concerning the Sino-US relationship. Niu Jun , a professor of international relations at Peking University, said Yang's views were shared by mainland hawks. 'They are not new, but it's just that the media hasn't reported this before. And this time for some reason this got reported,' he said. Although the Sino-US relationship has remained the most crucial tie for both countries, critics have noted that the mainland has become increasingly assertive on bilateral and international matters, thanks to growing economic clout. Niu said trade frictions could be a bigger issue in bilateral ties this year than the arms sale. Under the contract announced by the US Department of Defence, Taiwan will get an unspecified number of Patriot missiles. But the deal that was announced yesterday appeared to be of a smaller scale than had previously been suggested. Shi Yinghong , an expert on Sino-US relations from Renmin University, described this as a signal of Washington's effort to minimise the impact on ties with Beijing. Taiwan has been asking for F-16 fighter jets, which the US had withheld in 2008 after strong protest from the mainland.