Amid reports of frequent incursions by Chinese forces into Sikkim, Beijing on Thursday described the festering boundary row with India as a "very sensitive issue".

However, it noted that the two sides had agreed that their strategic cooperation in other areas should not be affected by the decades-old row.

"Tremendous changes" had taken place in Sino-India relationship compared to the past and a strategic partnership had been agreed to, Chinese Vice Foreign Minister Wu Dawei told reporters in Beijing. He said the leaders of the two countries had taken a strategic view of the relationship.

Both sides, Wu said, had agreed that they need to work together to maintain peace and tranquility in the boundary area and "strategic cooperation between China and India in other areas should not be affected by the boundary question."

"China and India have a common border of over 4,000 kms. This is a very sensitive issue," Wu said.

He said "people were collecting firewood or chopping trees, either Indians in China or Chinese in Indian territory. When this happened in the past, they would be arrested because people thought they were crossing the border illegally."

The troops along the two sides of the border also never interacted with each other in the past, "whether out of animosity or not, I cannot say. But tremendous changes have taken place in the China-India relationship," Wu said.

"A strategic partnership has been agreed to and the leaders of the two countries have taken a strategic and long term view of our relationship," he said.

Wu's passing remarks about Sino-India ties came during his media briefing on the agreement reached between Japan and China on Wednesday to jointly develop part of the gas field in the East China Sea.

It was a path breaking accord on a long-standing row between the two sides, whose relations were marked by tensions and disputes over wartime history, territory and East China Sea but have been warming up in recent years.

China and Japan have differences over Diaoyu island in the East China Sea with Beijing maintaining that it was an inalienable part of China since ancient times and that it holds indisputable sovereignty over it.

"This is an issue left from history. So we should seek to resolve them properly in the course of history," Wu said.

In the context of China-Japan ties, he also referred to a remark of late Chinese paramount leader Den Xiao Ping that "perhaps this generation is not smart enough to resolve these issues but the next generation will be smarter and they will work out a solution that would be mutually acceptable."