At least 20 Indian soldiers were killed in a clash with Chinese forces in Ladakh in the disputed Kashmir region, Indian officials say.
The incident follows rising tensions, and is the first deadly clash in the border area in at least 45 years.
The Indian army initially said three of its soldiers had been killed, adding that both sides suffered casualties.
But later on Tuesday, officials a number of critically injured soldiers had died of their wounds.
India's external affairs ministry accused China of breaking an agreement struck the previous week to respect the Line of Actual Control (LAC) in the Galwan Valley.
According to the Indian statement, "a violent face-off happened as a result of an attempt by the Chinese side to unilaterally change the status quo there".
China did not confirm any casualties, but accused India in turn of crossing the border onto the Chinese side.
Chinese foreign ministry spokesman Zhao Lijian said India had crossed the border twice on Monday, "provoking and attacking Chinese personnel, resulting in serious physical confrontation between border forces on the two sides", AFP news agency reported.
Local media outlets reported that the Indian soldiers had been "beaten to death" but there was no confirmation from the military. China's Global Times newspaper reported that "solemn representations" had been made with India over the incident.
The LAC is in reality poorly demarcated. The presence of rivers, lakes and snowcaps means the line can shift. The soldiers either side - representing two of the world's largest armies - come face to face at many points.
Both sides insist no bullet has been fired in four decades, and the Indian army said on Tuesday that "no shots were fired" in this latest skirmish.
There have been tense confrontations between the two nuclear powers along the border in recent weeks.
India has accused China of sending thousands of troops into Ladakh's Galwan valley and says China occupies 38,000sq km (14,700sq miles) of its territory. Several rounds of talks in the last three decades have failed to resolve the boundary disputes.
The two countries have fought only one war so far, in 1962, when India suffered a humiliating defeat.
In May, dozens of Indian and Chinese soldiers exchanged physical blows in a clash on the border in the north-eastern state of Sikkim. And in 2017, the two countries clashed in the region after China tried to extend a border road through a disputed plateau.
There are several reasons why tensions are rising now - but competing strategic goals lie at the root, and both sides blame each other.
India has built a new road in what experts say is the most remote and vulnerable area along the LAC in Ladakh. And India's decision to ramp up infrastructure seems to have infuriated Beijing.
The road could boost Delhi's capability to move men and materiel rapidly in case of a conflict.
India also disputes part of Kashmir - an ethnically diverse Himalayan region covering about 140,000sq km - with Pakistan.-BBC
