India tightened security on the border with Pakistan on Monday after heavily armed men stormed a police station in the northern frontier state of Punjab, killing six people and wounding several others.

Armed police exchanged fire with the gunmen, who were holed up in the police station in Gurdaspur district more than eight hours after the assault began at about 5 am (2330 GMT Sunday), officials said.

Gunshots were heard on television as security forces in red turbans surrounded the building in the town of Dinanagar, about 15km from the international border. Soldiers were also deployed, at least one armed with a shoulder-mounted rocket launcher.

"We have been able to limit (the attack), they are surrounded, they are holed up in the police station. We are on top of the situation," said Harcharan Bains, an adviser to Punjab's chief minister.

Police sources said the attackers had entered India from Pakistan a couple of days ago in the troubled state of Jammu and Kashmir, a short distance to the north. Jitendra Singh, a junior minister in Prime Minister Narendra Modi's office, said he did not rule out Pakistan's involvement.

"There have been earlier also reports of Pakistan infiltration and cross-border mischief in this area," said Singh, whose constituency in the Jammu region borders Gurdaspur.

Attacks on security installations by militants dressed as soldiers or police are common in Jammu, but Monday's was the first such operation in Punjab in 13 years, according to data from the South Asia Terrorism Portal, which tracks militant violence.

India's Federal Home Minister Rajnath Singh said he had spoken to the head of India's Border Security Force and "instructed him to step up the vigil" on the border.

"The situation is under control," Singh told reporters.

Five bombs were also found on a railway track in the state, suggesting an attempted coordinated attack, around the time India is marking the anniversary of a near-war with Pakistan in northern Kashmir in 1999.

The dead included four civilians and two policemen, said HS Dhillon, a senior Punjab police officer. Some others were wounded, he said. A suspected militant was also killed, but his body had not yet been recovered, another official said.

The group of about five attackers came in a white Maruti-Suzuki car, dressed in army uniforms, said Bains, the adviser to the Punjab chief minister. The attackers took the vehicle at gunpoint from a roadside "dhaba" restaurant, another local politician told Reuters.

Television footage showed the car with its windshield peppered with bullet holes, and broken glass and bullet casings on the passenger seat. What appeared to be improvised explosive devices on railway tracks were also shown.

DEADLY INSURGENCY

India fought a deadly Sikh insurgency in Punjab in the 1980s that peaked with the assassination of Prime Minister Indira Gandhi at the hands of her Sikh bodyguards in 1984.

That attack was in retaliation to her decision to order the army to flush out militants from the Golden Temple in Amritsar, the holiest shrine of the Sikh community.

Sikh militant groups were demanding an independent homeland for minority Sikhs at the time, which they called Khalistan.

"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," said Ajai Sahni, executive director of the Institute for Conflict Management in New Delhi.

Hasan Askar Rizvi, a political analyst in Pakistan, rejected Sahni's theory. "There is no information on this side to show support of Khalistan movement," he said. "You can't support a movement that doesn't exist on the ground (in India)." - Reuters