India acknowledged for the first time Thursday that it has matched China in massing troops at their contested Himalayan border region after a deadly clash this month.

But India's foreign ministry accused China of causing the tensions by starting military deployments, and warned relations between the world's two most populous nations could be undermined if the standoff continues.

The neighbours have blamed each other for a June 15 battle in the Ladakh region in which 20 Indian soldiers were killed while China suffered an unknown number of casualties.

While each has said it wants to de-escalate the territorial showdown, India's foreign ministry spokesman Anurag Srivastava said "both sides remain deployed in large numbers in the region, while military and diplomatic contacts are continuing".

Srivastava said "Chinese actions" on the unofficial border, known as the Line of Actual Control (LAC), led to this month's deadly fight with rocks and batons. No shots were fired.

"At the heart of the matter is that since early May, the Chinese side has been amassing a large contingent of troops and armaments along the LAC," the spokesman charged.

He added that China had obstructed India's patrols in contravention of accords made to avoid skirmishes between their armies, who fought a border war in 1962 and have regularly clashed since.

An Indian fighter jet flies over Leh, the joint capital of the union territory of Ladakh, on June 25, 2020, part of a show of strength after a border showdown between Delhi and Beijing
An Indian fighter jet flies over Leh, the joint capital of the union territory of Ladakh, on June 25, 2020, part of a show of strength after a border showdown between Delhi and Beijing AFP / Tauseef MUSTAFA

Srivastava said Chinese forces had built "structures" on the Indian side of their demarcation line in the Galwan Valley in Ladakh where the high-altitude battle was fought.

"While there have been occasional departures in the past, the conduct of Chinese forces this year has been in complete disregard of all mutually agreed norms," the spokesman said.

India had "to undertake counter deployments" because of the Chinese buildup, he said.

Military commanders have held talks and their foreign ministers have also discussed ways to end the showdown.

"Peace and tranquility in the border areas is the basis of our bilateral relationship," said Srivastava, demanding that China follow up on its pledge to cool tensions.

"A continuation of the current situation would only vitiate the atmosphere for the development of the relationship."

China has accused Indian forces of causing the June 15 battle by attacking its troops.

Beijing has also called on India "to immediately stop all infringing and provocative actions."

China India