China slams the U.S. for interfering in Arunachal Pradesh border dispute with India

China slammed the United States for interfering in its border dispute with India, after Washington said it recognized the disputed Arunachal Pradesh as a part of Indian territory.

"China strongly deplores and firmly opposes this," Chinese Foreign Ministry spokesperson Lin Jian said Thursday at a press conference in Beijing. "The China-India boundary question is a matter between the two countries and has nothing to do with the US side."

The response comes after the U.S. on Wednesday rejected China's "unilateral attempts" to advance its territorial claims, weighing in on a spat between New Delhi and Beijing after Prime Minister Narendra Modi inaugurated a tunnel in the Indian state of Arunachal Pradesh.

It is the latest in an escalating dispute between neighboring India and China, which share a 3,500 kilometer border.

China, which refers to the territory as Zangnan, claims Arunachal Pradesh is part of southern Tibet. India rejects those claims, stating Arunachal Pradesh has always been a part of India.

On Wednesday, the U.S. State Department weighed in on the matter.

"The United States recognizes Arunachal Pradesh as Indian territory and we strongly oppose any unilateral attempts to advance territorial claims by incursions or encroachments, military or civilian, across the Line of Actual Control," spokesperson Vedant Patel said.

The LAC is a demarcation that separates India-controlled territory from that controlled by China.

China on Thursday reiterated its claims over Arunachal Pradesh, saying: "Zangnan has always been China's territory, a basic fact that is undeniable."

"It is known to all that the US has consistently spared no efforts to provoke and take advantage of other countries' conflicts to serve its selfish geopolitical interests," Lin added.

U.S. support for India in the border dispute is a "reflection of consistent U.S. efforts to fully align itself with India in its competition with China," Michael Kugelman, director of the South Asia Institute at the Wilson Center told CNBC.

Kugelman pointed out that the U.S. typically refrains from commenting on some Indian border disputes, such as the one with Pakistan over Kashmir.

But in this case, he said, Washington is signaling its solidarity with New Delhi — "in the same way that it has made efforts, including through intelligence-sharing, to help India deter Chinese aggressions on its northern border."

Earlier this month, Indian Prime Minister Narendra Modi inaugurated the "Sela Tunnel" — the world's longest bi-lane tunnel  built at an altitude above 13,000 feet, which is located in the Indian state of Arunachal Pradesh, triggering sharp responses from Chinese officials.

Border tensions between India and China have risen in recent years. In a major escalation in 2020, a clash between the two sides killed 20 Indian soldiers and four Chinese troops. Last year, China renamed 11 places in Arunachal Pradesh, a move strongly opposed by India.  

Senior Colonel Zhang Xiaogang, deputy director general of the Information Office of China's Ministry of National Defense said in a statement days after the road tunnel inauguration that "China never recognizes and firmly opposes India's illegal establishment of the so-called 'Arunachal Pradesh.'"

Last week, China's defense ministry reiterated its claim over the northeastern state of Arunachal Pradesh.

India's foreign ministry this week responded to Zhang's comments, saying Arunachal Pradesh "was, is and will always be an integral and inalienable part of India."

Randhir Jaiswal, India's foreign ministry spokesperson said in an official statement that the Chinese Defense Ministry made "absurd claims" over the Indian State of Arunachal Pradesh and "repeating baseless arguments in this regard does not lend such claims any validity."

On the U.S. standing up for India, Harsh V. Pant, vice president for studies and foreign policy at Observer Research Foundation, a New Delhi-based think tank, said: "It shows how far India-U.S. relations have come."

"Even when it comes to the matter, which has been very sensitive, such as the India China border dispute, the U.S. today is openly standing with India," he told CNBC.

— CNBC's Naman Tandon contributed to this story.