The Indian army said Tuesday that 20 of its soldiers were killed in border clashes with Chinese troops, Reuters and The Associated Press reported, citing an army statement.
They are the first deaths in 53 years in the standoff between these two nuclear-armed powers.
The army originally reported that three Indian soldiers had died, but later said 17 additional soldiers had succumbed to injuries, according to the news agencies.
India and China, the world's two most populous countries, are locked in a territorial dispute along their mountainous 2,175-mile frontier.
No shots have been fired since 1975, with troops occasionally engaging in hand-to-hand scuffles and throwing rocks, as early reports indicated was the case Tuesday.
But experts worry that tensions are escalating between India — Prime Minister Narendra Modi is a friend of President Donald Trump's — and an increasingly assertive China.
In the past year, China has increased its nuclear arsenal from 290 to 320 warheads, and India from 130-140 to 150 warheads, according to the Stockholm International Peace Research Institute, or SIPRI.
The Indian army said in a statement late Tuesday that the two sides had "disengaged” from the disputed Galwan area after clashing overnight, according to The AP.
In an earlier statement, the Indian army said that one of its officers and two soldiers were killed in a "violent faceoff" in Galwan Valley, in the mountainous region of Ladakh.
Senior military officials from both sides were meeting to defuse the situation, it said.
China blamed India, whose troops it said "crossed the border for illegal activities and launched provocative attacks against Chinese personnel," the state-run tabloid newspaper, the Global Times, said citing a foreign ministry briefing.
"The Chinese side has lodged a strong protest and solemn representation to the Indian side, urging it to strictly restrain its front-line troops according to the consensus," it said.
Hu Xijin, the influential editor-in-chief of the Global Times, tweeted: "I want to tell the Indian side, don't be arrogant and misread China’s restraint as being weak. China doesn't want to have a clash with India, but we don't fear it."
The latest flare-up started last month with more reports of rock-throwing and culminating with thousands of troops now camped either side of the Galwan Valley.
The dispute dates to the 1860s, when British colonial rulers in India drew a border that was later disputed by China.
This flared into a conflict in 1962, when Chinese forces invaded and drove back their Indian counterparts in what is still remembered as a chastening defeat.
In the 1990s, both sides signed an agreement reaffirming their promise not to use military force.