At least 10 armed men disguised as policemen stormed a high-security prison in India's Punjab state on Sunday and freed a top Sikh militant commander and four others, officials said.

The attackers, who travelled in cars, stabbed and injured a guard at the main gate of Nabha Jail in Patiala district and fired several shots before fleeing with the inmates.

Two other guards were injured by the firing during the incident, India's second major jailbreak in less than a month.

A state-wide alert has been declared and special teams formed to hunt down the attackers, said H S Dhillon, Punjab director-general of police for law and order.

Police sealed all entry and exit routes in the state.

They identified one of the fugitives as Harminder Singh Mintoo, chief of the Khalistan Liberation Force (KLF) — a militant group fighting for a Sikh homeland in Punjab.

Mintoo was arrested in 2014 and is on trial for “terror attacks and funding”.

The four others are part of a local criminal gang and were on trial for murder.

Local media reports said the attackers numbered around 20 and that six prisoners had escaped.

Woman killed

A woman was killed late Sunday after police shot at her car when the driver allegedly failed to stop at a checkpoint on a highway, some 25 kilometres from Patiala district.

The woman was an artist travelling with her team to attend a wedding in a neighbouring state and had no connection to the jailbreak, police said.

“We are trying to find out what actually happened,” said one officer, requesting anonymity. Another person was injured.

Four officials, including the Punjab prisons chief and jail superintendent, have been suspended or sacked after the jailbreak, authorities said.

A reward of $36,000 has been offered for information on the escapees.

In October eight suspects awaiting trial escaped a prison in Madhya Pradesh state. They were gunned down hours later in a shootout, which many allege was staged by police.

A senior police officer said the swift assault on Sunday took guards by surprise as the attackers moved quickly through the complex, indicating they knew the jail layout.

“We are trying to figure out the connection between the gangsters and the militant leader. Did he flee by a fluke?” the officer told AFP on condition of anonymity.

Punjab was in the grip of an insurgency for almost three decades after several groups launched an armed campaign in the 1970s for an independent Sikh state to be known as Khalistan.

The violence peaked following the army's 1984 storming of the holiest Sikh shrine, Amritsar's Golden Temple, to flush out militants.

The military operation angered Sikhs, resulting in the assassination of Indian Prime Minister Indira Gandhi by her Sikh bodyguards. Thousands of Sikhs took up arms to avenge the sacrilege at the temple.

More than 20,000 people, mostly civilians, had died by the late 90s, when the insurgency waned.

But several Sikh groups remain committed to the Khalistan movement and dozens of alleged militants remain imprisoned.