The worst border clash between India and China in more than 40 years left 20 Indian soldiers, including a colonel, dead and dozens believed captured, Indian officials said on Tuesday, raising tensions between nuclear-armed rivals. Over 40 Chinese soldiers were killed in the dispute at the in the Galwan Valley in eastern Ladakh.

It’s probably the biggest military confrontation in over five decades.

China and India have accused each other of provoking fighting in the Ladakh region. India said China had tried to “unilaterally change the status quo”. Beijing accused Indian troops of “attacking Chinese personnel”.China’s Foreign Ministry was quoted by Reuters as saying India should not take unilateral actions or stir up trouble.

The two armies later held talks to try to defuse tensions.

Meanwhile, United Nations Secretary-General Antonio Guterres has expressed concern on the reports of violent clashes between India and China on the Line of Actual Control and urged both sides to exercise maximum restraint.

The US in a statement said it was closely monitoring the situation.

For the past several weeks, after a series of brawls along their disputed border, China and India have been building up their forces in the remote Galwan Valley, high up in the Himalayas.

Meanwhile, Rahul Gandhi attacked PM Narendra Modi over the issue. He tweeted, “Why is the PM silent? Why is he hiding? Enough is enough. We need to know what has happened. How dare China kill our soldiers?How dare they take our land?”

The LAC is 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.

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.