China admonished the United States on Monday for sending its ambassador in India to a contested stretch of land on the India-China border, warning that a third party's meddling would only complicate the dispute between Beijing and New Delhi.
China claims more than 90,000 sq km (35,000 sq miles) of territory disputed by India in the eastern sector of the Himalayas. Much of that forms the Indian state of Arunachal Pradesh, which China calls South Tibet.
U.S. Ambassador to India Richard Verma posted photos on his Twitter account on Oct. 21 of his recent trip to Arunachal Pradesh, thanking Indian officials for their "warm hospitality" and calling the region a "magical place".
Chinese Foreign Ministry spokesman Lu Kang said China was "firmly opposed" to the U.S. diplomat's actions, which he said would "damage the hard-earned peace and tranquility of the China-India border region".
"Any responsible third party should respect efforts by China and India to seek peaceful and stable reconciliation, and not the opposite," Lu told a regular press briefing.
"We urge the United States to stop getting involved in the China-India territorial dispute and do more to benefit this region's peace and tranquillity," he said, adding that China and India were handling the matter appropriately through talks.
A spokesman for India's Ministry of External Affairs was not immediately available for comment.
Disagreement between the nuclear-armed neighbours over parts of their 3,500-km (2,175-mile) border led to a brief war in 1962. The countries have moved to control the dispute, but repeated rounds of talks have failed to make much progress.
India says China occupies 38,000 square km (14,600 sq miles) of its territory on the Aksai Chin plateau in the west, and is also suspicious of China's support for its arch-rival, Pakistan.
Tensions occasionally flare over the disputed border.
In August, China was angered by India's plans to place advanced cruise missiles there.