The act of Indian border troops recently crossing into Chinese territory was described as "very serious" by a Chinese Foreign Ministry spokesperson Monday.

"The current situation is totally different from previous confrontations between the two countries' border troops, which happened at an undefined China-India boundary," spokesperson Geng Shuang said.

The latest crossing occurred at the Sikkim section of the China-India boundary that had long-been demarcated, Geng said, noting this section had been defined in the Convention Between Great Britain and China Relating to Sikkim and Tibet in 1890.

According to the convention, the border line commences at Mount Gipmochi in the east and follows the water-parting until it meets Nepal, Geng told a routine press briefing.

Geng said successive Indian governments had acknowledged the demarcation in writing and agreed to the border between Sikkim and Tibet.

Documents between the Chinese and Indian governments show former Indian Prime Minister Jawaharlal Nehru admitted several times that the Sikkim-Tibet border was defined by the 1890 convention, on behalf of the Indian government, Geng said.

In a letter to then Chinese Premier Zhou Enlai on March 22, 1959, Nehru said the border of India's protectorate Sikkim and China's Tibet was defined by the 1890 convention and marked by both sides on the ground in 1895.

In another letter from Nehru to Zhou on Sept 26 the same year, Nehru repeated the information, adding there was no dispute regarding the border between Sikkim and Tibet.

"These documents show India's current move runs counter to the consistent position of the Indian government," Geng said.

Geng said that Indian border troops overstepped the mutually recognized boundary line and crossed into the Chinese territory, and that India's move violated the 1890 convention, as well as the basic norms of the UN Charter and international laws.

China has lodged protest to India several times, asking the country to observe the existing boundary convention, respect China's territorial sovereignty, and immediately withdraw its border troops that crossed into China, Geng said.