China on Monday said India was using Bhutan to "cover up" the illegal entry by Indian troops into "Chinese territory" and demanded their immediate withdrawal.

China said India's first Prime Minister Jawaharlal Nehru had accepted the 1890 treaty, a pact Beijing has sworn by to claim Donglong, which is disputed between China and Bhutan.

The Chinese Foreign Ministry said India's action went against the "basic principles of UN charter and the international law and infringed upon Chinese territorial integrity".

Foreign Ministry spokesperson Geng Shuang cited letters by Nehru to then Chinese Premier Zhou Enlai.

China and India have refused to budge from their positions over the standoff between their troops in Donglong, which the Chinese claim as their territory and which India calls a disputed region between China and Bhutan.

Tensions have been building up between the two Asian giants. A Chinese expert said that war between the two countries could not be ruled out.

"In order to cover up the illegal entry by Indian troops into the Chinese territory, the Indian side even wants to infringe upon Bhutan's sovereignty that is futile," said the Foreign Ministry spokesperson.

China says India should not interfere in the dispute between Thimpu and Beijing. However, Bhutan has opposed China's building the road in the disputed Donglong.

"We have no objection to normal bilateral relations between India and Bhutan but firmly opposed to the Indian side infringing on Chinese territory using Bhutan as an excuse.

"The Bhutan side does not know previously that the Indian troops entered the Doklam area, which is not in line with what is claimed by the Indian side," Geng said.

Asked what he thought about Indian Defence Minister Arun Jaitley's remarks that the India of 2017 was not the India of 1962, Geng said: "To some extent, he is right in saying that India in 2017 is different from the India in 1962, just like China is also different."

Last week, after China had warned India to learn from its military debacle of 1962, Jaitley had said that the India of 2017 was not the India of 1962.

"Donglong is located on the Chinese side of the boundary and belongs to China. By entering into Chinese territory and obstructing Chinese military troops' normal activities, the Indian side has violated existing convention," Geng said.

"We have noted the statement from the Indian side; (it) evaded the 1890 convention between Great Britain and China and Tibet.

"Indian Prime Minister Nehru on behalf of the Indian government explicitly recognised that the 1890 convention has defined the boundary."

Geng cited two letters by Nehru to Zhou En Lai in 1959, stating that 1890 defined the border as demarcated.

"The action taken by India is undoubtedly a betrayal of the consistent position held by successive Indian governments," Geng said. "We request India to respect the treaty and pull back the border troops immediately."