Some of the world's best known China scholars have signed an open letter to President Jiang Zemin appealing for the early release of Hong Kong academic Dr Xu Zerong, who was jailed in Shenzhen last month for 13 years. City University professor Li Shaomin and US-based academic Gao Zhan - who were jailed on the mainland last year but later released - were among the 340 signatories. The letter was released yesterday by St Antony's College of Oxford University where Xu spent 10 years conducting research. 'We accordingly urge you, Mr President, to look into the case of Dr Xu and to bring about his early release,' the letter said. 'We also urge your Government to affirm its commitment to promoting and protecting academic freedom and human rights in China and thus provide a secure basis for the continuation of scholarly exchanges between your country and those to which we belong.' Other signatories to the letter included journalists and politicians such as Harold Koh, a former assistant secretary of state under the Clinton administration, and several members of Britain's House of Lords. Xu was sentenced to 10 years' jail by Shenzhen Intermediate People's Court last month for illegally obtaining confidential Chinese government documents in 1992 and publishing them in his thesis. He received an additional three years for publishing books and periodicals for profit without authorisation. Steve Tsang, a fellow of St Antony's College, said Xu's case raised serious questions about academic freedom and the training of Chinese students overseas. Mr Tsang said Xu's research in 1992 covered the Korean War, and his thesis, now in an Oxford University library, could not have contained information that could endanger China's state security. He said unless Beijing provided more information on Xu's case, the sentencing would cause more alarm among China scholars. Xu, a Hong Kong permanent resident, was born in Sichuan in 1954 and obtained a doctorate from Oxford in 1999.