A Foreign Ministry official described on Monday the trespassing by Indian troops across the China-India border at the Sikkim section as "essentially different" to previous standoffs with Chinese border troops, as this time they crossed a clearly demarcated borderline between the two countries.

This incident is "very severe" as it goes against a "legally binding" boundary treaty, as well as the Charter of the United Nations and the basic principles of international law, ministry spokesman Geng Shuang said.

Photos taken on June 18, which the ministry has published on its website, show Indian troops crossing the demarcated Sikkim section into the Chinese territory of Doklam.

Geng said earlier that the Indian troops obstructed Chinese border troops' normal activities in the area, and China had "taken proportionate measures in response".

The Sikkim section of the China-India boundary has been defined by the Convention Between Great Britain and China Relating to Sikkim and Tibet, which was signed in 1890, Geng reiterated on Monday.

Since the establishment of the People's Republic of China and the independence of India, governments of both countries have recognized the validity of the treaty, and have repeatedly confirmed it in a series of official documents and meetings, Geng said.

China has lodged protest over the incursion to India many times, asking it to observe the treaty, respect China's territorial sovereignty and move its troops back onto its side of the boundary, the spokesman said.

wangqingyun@chinadaily.com.cn