Heavy gunfire was heard from inside an Indian airbase near the Pakistani border as a siege on the base spilled into its third day.

Indian officials vowed on Monday to kill any remaining attackers inside the base in Pathankot, with at least one gunman thought to still be holed up after a two-day siege had left seven soldiers and six attackers dead.

On Sunday, Home Secretary Rajiv Mehrishi told reporters that the last of the fighters had been cornered and would be "neutralised" soon.

Al Jazeera's Faiz Jamil, reporting from New Delhi, said that "some of the gunmen may have been hiding in some of the bunkers or the forested areas surrounding the large military base" before launching the attack.

"Indian officials aren't sure themselves, but because of the terrain it makes it difficult to determine. Officials are staying tight-lipped about the operation, and don't want to say how many gunmen exactly are inside because they aren't sure," Jamil said.

Punjab attack threatens India-Pakistan peace progress

The attack on the Pathankot air force base started before dawn on Saturday and is considered by some as an attempt to undo recent improvements in the relationship between archrivals India and Pakistan.

It comes a week after Narendra Modi became the first Indian prime minister in 12 years to visit Pakistan.

No group has claimed responsibility for the attack.

Troops engage attackers

Mehrishi said that Indian authorities were alerted about a potential attack in Pathankot, and that aerial surveillance at the base spotted the suspected fighters as they entered the compound.

He said that they were engaged by Indian troops and were kept away from the base's aircraft and military equipment.

Since Saturday morning, the base has been swarming with air force commandos, troops from India's National Security Guard and local police.

A senior air force officer, Air Marshal Anil Khosla, told reporters in New Delhi that the base would not be declared fully secured until the entire area is checked by troops.

The sprawling Pathankot air force base is spread over several kilometres, including some forested sections. It houses a fleet of India's Russian-origin MiG-21 fighter jets and Mi-25 and Mi-35 attack helicopters, along with other military hardware.

The base is on the highway that connects Indian-administered Jammu and Kashmir with the rest of the country.

The Himalayan region of Kashmir is divided between India and Pakistan, but is claimed in its entirety by both. Rebels in the Indian-administered portion of Kashmir have been fighting since 1989 for independence or merger with Pakistan.

Source: Al Jazeera and agencies