A seven-week military standoff between India and China escalated into a deadly conflict along their contested Himalayan border for the first time in more than four decades, signaling a sharp deterioration in ties between the two regional giants.

It’s unclear what sparked the clashes or how many died in Monday’s violence — so far India has confirmed 20 of its troops were killed. Zhang Shuili, a military spokesperson in China’s western battle zone command, said in a statement there were casualties on both sides, without elaborating.

Just two days ago it appeared efforts to lower the temperature on both the diplomatic and military fronts were working, with the two sides indicating they were pulling back forces while talks continued.