China lodged an official protest with India on Thursday after charging that an Indian drone had "invaded" its airspace before crashing, months after the two sides ended a tense border standoff.
Beijing said the incident occurred "recently" at the border separating India's northeastern Sikkim state and China's Tibet region, but it did not say exactly where and when.
India's army said the unmanned aerial vehicle was on a "regular training mission" when ground control lost contact with it "due to some technical problem" and it crossed over the demarcation line.
The Chinese foreign ministry urged India to "stop the activities" of drones near the border after the UAV "invaded" its airspace.
"The action of the Indian side violated China's territory and is not conducive to the peace and tranquility of the border area," foreign ministry spokesman Geng Shuang told a regular news briefing.
"China is dissatisfied with this and lodged solemn representations with the Indian side," he said, referring to the official diplomatic protest.
A Chinese army official said earlier that border troops "took a professional and responsible attitude" and carried out identification verification of the device.
"We will earnestly fulfil our mission of duty and firmly defend the sovereignty and security of our country," the deputy director of the Chinese army's western theatre combat bureau, Zhang Shuili, said in a statement.
The Indian army said the country's border security personnel "immediately alerted" their Chinese counterparts to locate the UAV.
"The exact cause of the incident is under investigation," Indian army spokesman Colonel Aman Anand said in a statement.
"The matter is being dealt with in accordance with the established protocols through institutional mechanisms to deal with situations along the India-China border areas."
The drone incident follows a summer standoff in a Himalayan area where Tibet, Sikkim and Bhutan meet.
In August, the two nations pulled back their troops to resolve the tense deadlock over the area, which is claimed by both China and Bhutan, an ally of India.
The dispute began in mid-June after Chinese troops started building a road on the Doklam plateau, known as Donglang in Chinese.
India has an army base nearby and moved soldiers into the flashpoint zone to halt the work, prompting Beijing to accuse it of trespassing on Chinese soil.
After both sides withdrew, India's army chief said in September that his country could not afford to be complacent and must be prepared for war.
"As far as our northern adversary is concerned, flexing of muscles has started," General Bipin Rawat said at a think tank event in New Delhi, in reference to China.
"The salami slicing, taking over territory in a very gradual manner... testing our limits of threshold is something we have to be wary about and remain prepared for situations which could gradually emerge into conflict," the army chief said.
Rawat said India also has concerns that its arch rival Pakistan -- an ally of China -- could take advantage of the tensions.
India and China went to war in 1962 over the state of Arunachal Pradesh.
The latest episode has fed into a broader competition for regional influence between the two Asian powers.
The two emerging economies both have large populations and a growing middle class.
China has invited India to join President Xi Jinping's new "Silk Road" project to revive ancient trade routes from Asia to Europe and Africa.
But the proposed economic corridor has alarmed India, partly because one of the links cut through Pakistan-administered Kashmir, disputed territory that New Delhi claims is illegally occupied.