Sikkim is 'an enduring issue left over from history' and cannot be resolved overnight, China said on Monday.

"We have to respect history. We have to take into consideration realistic factors," Chinese foreign ministry spokesperson Kong Quan said at his scheduled biweekly briefing attended by Indian journalists covering Prime Minister Atal Bihari Vajpayee's visit to China.

In an agreement on border trade, India and China had on Monday decided to open trade routes through Sikkim and Tibet. The Indian media interpreted this as recognition by China that Sikkim is a part of India.

The agreement and a joint declaration will be issued on Tuesday.

The Indian media had reported that the Indian government has, in these documents, acknowledged Tibetan Autonomous Region as a part of the People's Republic of China.

The Chinese spokesman said credit has to be given to Vajpayee and his Chinese counterpart Wen Jiabao for whatever is in the formulation on Tibet.

India has admitted that TAR is an 'inalienable' part of China, he added.