The Indian army says 20 of its soldiers have been in killed in border clashes with Chinese troops.
The development marks a major escalation following a stand-off between the two sides in the western Himalayas lasting several weeks.
The Indian army said 17 of its troops who were critically injured in the fighting in the Galwan Valley on Monday night have now died.
It follows the deaths of an officer and two soldiers earlier. They have been named as Colonel B Santosh Babu, Havildar Palani and Sepoy Ojha.
The violence happened along the Line of Actual Control (LAC), a disputed border that separates the two nuclear powers high up in the Himalayan mountains.
Indian and Chinese troops fought with iron rods and stones, and no shots were fired, according to an Indian government source, Reuters reported.
Although tensions have been high recently, with both sides accusing the other of incurring on to sovereign territory, these are the first fatalities between the two sides for three decades.
In a statement, the army said: "Indian troops who were critically injured in the line of duty at stand-off location and exposed to sub-zero temperatures in the high-altitude terrain have succumbed to their injuries, taking the total that were killed in action to 20.
"Indian army is firmly committed to protect the territorial integrity and sovereignty of the nation."
Indian and Chinese troops have disengaged in the areas where the clashes took place, the statement said.
The Indian army earlier said: "During the de-escalation process under way in the Galwan Valley, a violent face-off took place yesterday night with casualties on both sides."
The Indian government source claimed the People's Liberation Army had turned on a group of Indian soldiers.
"They attacked with iron rods, the commanding officer was grievously injured and fell, and when that happened, more soldiers swarmed to the area and attacked with stones," said the source.
The Chinese side brought in reinforcements and the brawl went on for a couple of hours, the source added.
The Chinese military has not yet commented officially on the incident but reports are emerging that they also sustained casualties, including five deaths.
Blaming Indian forces for the incident, the foreign ministry in Beijing, said: "Indian troops seriously violated our consensus and twice crossed the border line for illegal activities and provoked and attacked Chinese personnel which lead to serious physical conflict between the two sides.
"China has lodged strong protest and representation with the India side, and once again we solemnly ask the India side to follow our consensus and strictly regulate its front line troops and do not cross the line and do not stir up troubles or take unilateral moves that may complicate matters.
"We both agreed to resolve this issue through dialogue and consolation and make efforts for easing the situation and upholding peace and tranquillity in the border area."
Indian officials have accused Chinese troops of crossing the border at three different points in recent weeks, and setting up tents to claim ground.
This resulted in a massive build-up of troops on both sides, but high-level talks had been taking place when this latest incident took place.
The LAC was drawn up after a 1962 war between the two sides, creating one of the world's longest land borders.
Because of the mountainous landscape it is hard to define and has been the subject of regular diplomatic dispute, although physical clashes have mostly been small-scale.
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