Chinese troops reportedly crossed the Line of Actual Control at Doka La in Sikkim earlier this week and destroyed Indian Army bunkers.
The entire border is disputed, with a salient of Chinese territory — the Donglang region — jutted out between Sikkim and Bhutan.
"Donglang Region is part of China's territory. China's road-building activities in Donglang on its own territory are totally reasonable and understandable," said Chinese Foreign Ministry spokesman Lu Kang on Thursday (June 29).
Indian troops reportedly formed a human wall to prevent any further incursions by the PLA.
Sikkim is a tiny state which is wedged between Nepal and Bhutan. It was an independent Buddhist kingdom until 1975, until it was swallowed up by India.
General Bipin Rawat was appointed as chief of the Indian Army in December and has ordered the building of movable modern bunkers along the border shared with China.
But the construction work has been seen as provocative by the Chinese military, which is particularly sensitive about the border with Tibet, which it annexed in 1950.
General Rawat arrived in Sikkim on Thursday to take stock of operations and talk to his top commanders.
Earlier he had said India was ready for a war on "two and a half fronts," suggesting India could take on Pakistan, China and rebels in Kashmir.
China has warned India and described General Rawat's comments as "irresponsible."
"Such rhetoric is extremely irresponsible. We hope that the particular person in the Indian Army could learn from history lessons and stop such clamoring for war," said Colonel Wu Qian, a PLA spokesman, on Thursday.
Colonel Wu's remarks have been taken as a reference to the 1962 Sino-Indian War when India's then Prime Minister Jawaharlal Nehru, urged on by the jingoistic Indian press, received a bloody nose when the PLA inflicted a heavy defeat in the Aksai Chin region.
"There are not many instances in history where one country, that is India, had gone out of her way to be friendly and cooperative with the Chinese government and people, and to plead their cause in the councils of the world, and then for the Chinese government to return evil for good and even go to the extent of committing aggression and invading our sacred land," he said.
But the Indian Army learned a lot of lessons from the war and three years later it put in a considerably better performance in the war with Pakistan.