CHINESE President Mr Yang Shangkun yesterday called on Hongkong authorities to change their attitude and resume co-operation with China. Mr Yang, who is tipped to retire in April, passed on his appeal through the China International Broadcasting Station as part of his New Year message to mainland and overseas Chinese. Mr Yang said the days for Hongkong and Macau to revert to Chinese sovereignty were drawing near. ''Some recent actions by the British Hongkong authorities have constituted a hindrance to Hongkong's smooth transition,'' he said. ''This is something which the Chinese people, including the Hongkong compatriots, do not want to see. ''I hope the British Hongkong authorities can change their attitude and go back to co-operation,'' he said. The vice-chairman of the United Democrats of Hongkong, Mr Yeung Sum, said he could not tell from Mr Yang's remarks if China had toned down its opposition to the political reforms proposed by the Governor, Mr Chris Patten. ''The only thing is they [Chinese leaders] did not attack, use strong words or call on Mr Patten to withdraw his proposals in the message,'' he said. Also speaking in Beijing, the deputy director of the Hongkong and Macau Affairs Office under the State Council, Mr Chen Ziying, said China would observe the Sino-British Joint Declaration regardless of what happens in Hongkong in the coming year. ''I remember some British leaders had said that the Sino-British co-operation was very important to the maintenance of Hongkong's stability and prosperity,'' he told a television interview. ''I do not know if this statement is still valid today. The beginning of 1993 means a year nearer to 1997. ''No matter what will happen in Hongkong, China will still observe the Joint Declaration and China is confident and capable of implementing the Basic Law and the 'one country, two systems' in the Hongkong Special Administrative Region,'' he said. The vice-director of the local branch of the New China News Agency, Mr Zhang Junsheng, said Sino-British relations would not improve in 1993 unless Britain went back to the track of the Joint Declaration. He said he hoped that both sides would treasure and preserve the achievements that Hongkong had made, and not destroy them.