NEW DELHI — Shelling and gunfire intensified on Wednesday on the de facto border between India and Pakistan in the Kashmir region, killing nine civilians on a bus one day after the Indian Army promised retribution for what it said was the killing of three of its soldiers.

Pakistan said Indian troops fired on a bus in the Neelam Valley on Pakistan’s side of the Line of Control in the disputed Kashmir region, killing the nine passengers and seriously wounding nine others. The Indian military also fired on rescue workers in an ambulance trying to reach the wounded, Pakistan said.

In other violence reported on Wednesday, the Indian military also killed three Pakistani soldiers, including a captain, Pakistan said, and Pakistani forces retaliated, killing seven Indian soldiers.

A high-level Pakistani diplomat, Deputy High Commissioner Syed Haider Shah, called the violence “a serious escalation of the situation” and a “grave breach of international and humanitarian law.”