Gunmen, dressed in army uniforms, stormed into a police station on Monday in the northern Indian state of Punjab, killing at least six people.
The attack began in the early morning hours as a group of about five attackers first opened fire on a bus near the Punjab-Jammu Kashmir border, then seized the police station in the town of Dinanagar.
All of the policemen inside the building could have been killed, local media reported. Police cordoned off the area and gunshots could be heard on television as the clashes inside the building continued, Reuters reported.
According to the Punjab police officials, four civilians and two policemen have been killed while some others were injured. Police teams also found five bombs on a railway track in the state.
"They fired indiscriminately on the way to the police station, killed at least one policeman and injured five," Harcharan Bains, an advisor to Punjab's chief minister, told Reuters.
India's Border Security Force was ordered "to step up the vigil on India-Pakistan border," Federal Home Minister Rajnath Singh said in a Twitter message.
"I am confident that the situation will soon be brought under control," he said.
Punjab witnessed a deadly Sikh insurgency in the 1980s which demanded an independent for Sikhs. Former Prime Minister Indira Gandhi was assassinated by her Sikh bodyguards in 1984 in retaliation to an army operation against militants based in the Golden Temple in Amritsar, the holiest shrine of the Sikh community.
"This is probably linked to the movement of the 1990s, these are the Khalistanis who have done this. The remnants of that movement are still being hosted and protected in Pakistan. That is who I assume would be behind it eventually. Directions would have come from there," Ajai Sahni, executive director of the Institute for Conflict Management in New Delhi, told Reuters.
The attack also coincides the anniversary of a near-war with Pakistan in northern Kashmir in 1999. India's media claimed the gunmen sneaked into the province from the Indian held Jammu Kashmir region.
Kashmir is divided between India and Pakistan and the two countries have fought over the region since the partition of India in 1947. China also claims a part of the region.
Since 1989, different Kashmiri groups have been fighting for independence or unification with neighbouring Pakistan against Indian rule. There are tens of thousands of Indian army, paramilitary and police forces in the region, which is one of the most militarised areas in the world.
More than 100,000 Kashmiri people have been killed under the occupation, with thousands of people disappearing or being jailed.