Beijing on Thursday released two photographs showing Indian troops and at least two military bulldozers on what it said was the Chinese side of the border, India Today reported.
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The pictorial evidence, presented as proof of Indian army's 'transgression' into its territory, had the boundary highlighted with a red line.
"The Donglang region belongs to Chinese territory and it is very clear the conditions of the threshold where the areas are that were trespassed by the Indian troops," Chinese Foreign Ministry spokesperson Lu Kang said. "This is an undeniable fact that Indian troops have trespassed the boundary and is violating our historical boundary conventions and as well as promises by the Indian government."
According to the Chinese interpretation of events, Indian guards crossed into the Donglang region earlier in June and obstructed work on a road on a plateau, a Reuters report said.
The two sets of troops then confronted each other close to a highly strategic valley controlled by China that separates India from Bhutan - a close Indian ally - and gives China access to the so-called Chicken's Neck, a slither of land that connects India to its remote northeastern regions.
Indian media have reported that the dispute began when China removed an old Indian bunker. China says that unlike other parts of their shared border, the delineation of the frontier with Sikkim is settled, and it has the right to develop the area. The Nathu La Pass, which lies on the frontier between Sikkim and Tibet, was the site of a fierce border clash between Chinese and Indian troops in 1967.
Meanwhile, according to Reuters, India's army chief visited the mountainous state of Sikkim bordering China on the same day, where tensions have flared after Beijing accused New Delhi of sending troops into its territory and obstructing the construction of a road.
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Small incursions and troop stand-offs are common along other parts of China and India's contested 3,500 km frontier, but a flare-up near strategically positioned Sikkim is rare.
General Bipin Rawat arrived in Gangtok, the capital of the tiny Himalayan state, before heading off to visit troop formations. A spokesperson for the Indian army said the one-day trip was 'routine' and planned before the recent tensions.
Reports of the incursion and a confrontation between troops emerged as Prime Minister Narendra Modi arrived in Washington to meet US President Donald Trump on Monday, the latest in a line of stand-offs reported during diplomatic initiatives.
India has deepened its military ties with the United States in recent years, worrying China, which is also unhappy with India's refusal to participate in Beijing's multi-billion-dollar Belt and Road infrastructure initiative. India is concerned the project could cement China's dominance over Asia.