Three Indian soldiers have been killed along the de facto border with Pakistan in the disputed region of Kashmir, according to Indian army officials.
Army spokesman Colonel Rajesh Kalia said on Tuesday the attackers also mutilated the body of one soldier in Machil sector, in Indian-controlled Kashmir.
"Retribution will be heavy for this cowardly act," he said.
Another army officer, speaking on condition of anonymity in keeping with military rules, said the three soldiers were ambushed by a team of attackers who came from the Pakistani side.
Kalia did not specify if the attack was carried out by Pakistani soldiers or rebels fighting since 1989 against Indian rule in the Himalayan region.
However, an army official told it was carried out by the Pakistani soldiers.
"This border action was like a sneak attack by Pakistan soldiers," he said on condition of anonymity.
Kashmir has been divided between India and Pakistan since the end of British colonial rule in 1947. Both nuclear-armed rivals claim the territory in full and have fought two of three wars over the mountainous region.
The conflict has killed tens of thousands of people, mostly civilians.
In the past, the Indian army has blamed a combination of the Pakistani army's border action team and insurgents for carrying out operations along the so-called Line of Control (LoC), the de facto border dividing Indian and Pakistani-controlled regions.
Following Tuesday's attack, heavy cross-border firing and shelling began at several posts in Machil sector.
Pakistan's army insisted it only returned fire after coming under attack near the LoC. There was no response to the ambush allegations.
On Monday, four people, including two children, were killed and 10 others wounded in cross-border fire in Kashmir.
Tensions across the long-disputed border reached dangerous levels in September after India blamed Pakistani fighters for a raid on an army base that killed 19 of its soldiers.
India said it responded by carrying out "surgical strikes" across the heavily militarised border, sparking a furious reaction from Islamabad, which denied any attacks took place; according to a media report.