China is aware of the case of Wang Qi, who was detained when he crossed into India in 1963, Foreign Ministry spokesman Lu Kang said.
"We sympathise with what happened to him and will provide assistance to him. We believe under joint efforts and by respecting his will, the case will be solved properly," he said.
He has sought permission from New Delhi and Beijing to allow him to travel to China to meet his three brothers and two sisters. Born in a farmer family in Shaanxi province, he studied surveying and joined China's People's Liberation Army in 1960.
His is a complex case for both the sides as Wang - after being released in 1969 - settled in Tirodi village, where he married a local and has three children.
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In New Delhi, External Affairs Ministry spokesman Vikas Swarup yesterday said, "We are in touch with Ministry of Home Affairs to ascertain the details and how best it could be handled."
Wang's case has been highlighted in the Chinese media following a recent BBC television report on him. And, an Op-ed article in China's state-run Global Times said helping Wang return home would enhance mutual understanding and contribute to warming of ties between China and India.
"Although it's unclear whether Wang is a prisoner of war, it is inhumane to have isolated the elderly man from his family for such a long time," the article said.
However, the Op-ed piece did not mention Wang's marriage to the Indian woman.
"The Indian authorities should give their utmost attention to this case, actively communicating with the Chinese Embassy and improving administrative efficiency in processing this case, while the Chinese side should cooperate to provide the necessary documents pertaining to Wang that India requires," the article demanded.
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Wang was issued a passport by the Chinese Embassy in New Delhi in 2013 and applied for permission to travel to China in 2014, but since Wang's quest has made no headway due to "official procedures", the article said.
Wang has told the BBC he was "tasked with building roads for the Chinese army" and was captured when he "strayed erroneously" inside India's territory in January 1963.
"I had gone out of my camp for a stroll but lost my way. I was tired and hungry. I saw a Red Cross vehicle and asked them to help me. They handed me over to the Indian army," he said.