A group of gunmen has attacked an Indian Air Force (IAF) base in the northwestern state of Punjab that borders Pakistan, local officials said.
Four of the gunmen were killed in a shoot-out with security forces early on Saturday morning. Exchange of fire has ended and combing operation is under way at the base, located about 50km from the Pakistan border.
The air base has been cordoned off and heavy police deployment is in the area, with elite paramilitary force of National Security Guard (NSG) and the Guard Commando Force called in at the spot. 
Helicopters have also been dispatched to the IAF base for assistance in the operation.
It is believed that at least two Indian soldiers were also killed.
A senior Indian police officer said that red alert has been issued across Punjab state, where the incident occurred.
Al Jazeera's Faiz Jamil, reporting from New Delhi, said the attack began at around 3.30am local time (22:00 GMT) and that the attackers had entered the base wearing military uniforms.
"Officials are not sure yet who these gunmen are," he said, adding that officials were saying that the remaining attackers had been contained to a non-operational area of the base.
According to early accounts, the attack resembled a similar raid last year by gunmen on a border town in Punjab that killed nine people.
India blamed that attack on assailants who had infiltrated from Pakistan.
The latest attack is likely to deal a blow to attempts to revive political dialogue between the two nuclear-armed neighbours after Indian Prime Minister Narendra Modi made an impromptu visit to Pakistani counterpart Nawaz Sharif in late December.
One Indian home ministry official said Punjab and Jammu states were on high alert and all defence bases had been sealed.
"Attacking an air base is a serious security threat. The new strategy of the terrorists is to identify defence bases near the border and launch attacks," said the official, who was not authorised to comment on the record.