China has made a formal protest after accusing Indian border guards of crossing from Sikkim state into its Tibetan territory, China's foreign ministry said Tuesday.
India and China have long been embroiled in a bitter border dispute at both ends of the Himalayas, with the two countries accusing soldiers of crossing over into the other's territory.
"Our position to uphold our territorial sovereignty is unwavering," spokesman Lu Kang told a regular briefing, adding China has lodged "solemn representations" with India.
"We hope that the Indian side can work with China in the same direction and take actions to withdraw the personnel who have overstepped and trespassed into Chinese borders."
A statement posted on the Chinese defense ministry's social media account offered few details about the alleged incident, but said it had "seriously endangered peace and tranquility in the border areas".
The Indian military has "unilaterally stirred up trouble" by obstructing road building activities on the Chinese side of the border separating Sikkim and the Tibetan region, it said.
In apparent retaliation, China has blocked Indian pilgrims from crossing the border in the mountainous area "out of security concerns", the Chinese foreign ministry said.
Nathu La connects India to Hindu and Buddhist sites in the region and was the site of a fierce border clash between Chinese and Indian troops in 1967.
"For the upcoming actions we have to depend on what the Indian side will do. They have to take actions to improve the security situation," the foreign ministry's Lu said.
A spokesman for the Indian Army declined to comment but said the army would issue a statement on Tuesday. India's defense ministry did not immediately respond to a request for comment.
Indian media said China had turned back 50 pilgrims trying to cross into Tibet to visit a mountain revered as the home of the Hindu god Shiva. Authorities there normally allow the annual pilgrimage to go ahead.
India's foreign ministry spokesman said last week the pilgrims had experienced "some difficulties" in movement and these were under discussion with Beijing.
In an earlier statement, China's foreign ministry called on India to "immediately withdraw their border troops that have crossed the boundary, conduct a thorough investigation into this and safeguard peace and tranquility of the Sikkim section".
Tensions along the frontier rose in 2014 when Chinese soldiers moved into territory claimed by India, sparking a two-week military stand-off.
Hundreds of Indian and Chinese troops faced off on the de facto border known as the Line of Actual Control, which runs along the remote mountainous region of Ladakh in northwest India, overshadowing a visit by China's President Xi Jinping.
The neighbors, now nuclear-armed, fought a brief but bloody war in 1962 over the Indian state of Arunachal Pradesh in the eastern Himalayas, and are still embroiled in a bitter dispute over the territory.
Aside from the festering border issue, China's close ties with India's archrival Pakistan, where it is pursuing infrastructure projects under Beijing's global Belt and Road initiative, has also been a source of tension.
"China is prone to flexing its muscles on the border to send a message that says 'I am the big boy on campus'," said Samir Sara, vice president of the New Delhi-based Observer Research Foundation think tank.
Sara said India's rejection of the Belt and Road project and its opposition to Beijing's plans around the world "may have triggered this" latest row.
Chinese President Xi Jinping told Indian Prime Minister Narendra Modi earlier this month that the two countries should work to "appropriately" manage their differences.
A visit in April by Tibetan spiritual leader the Dalai Lama, who Beijing brands a separatist, to a region controlled by India but claimed by China also stoked tensions between the two countries.
The Indian government has since taken steps to cool tensions, rejecting an Australian request to take part in joint naval exercises with the United States and Japan last month to avoid agonizing China.
Modi is expected to visit China in September to attend a summit of the BRICS nations, which groups Brazil, Russia, India, China and South Africa.