NEW DELHI (AFP) - India on Monday (Dec 19) accused Pakistani militant leader Maulana Masood Azhar of masterminding an audacious attack on an air force base in January that led to a breakdown in relations between the two nuclear-armed nations.

Seven soldiers were killed in the attack on the Indian air force base in Pathankot, which New Delhi has said could not have been carried out without the help of the Islamabad government.

It blames the Pakistan-based Islamist group Jaish-e-Mohammed (JeM), founded by Azhar, who was released from an Indian prison in exchange for passengers of a hijacked Indian Airlines plane in 1999.

India's National Investigation Agency (NIA) filed formal charges on Monday naming Azhar, his brother Rauf Asghar and two other members of the banned group after concluding an investigation into the January attack.

"We have filed the charge sheet and further investigations will continue," an NIA official told AFP on condition of anonymity. "We have material evidence against the accused."

No arrests have yet been made over the Pathankot attack, a rare instance of militants targeting an Indian military installation outside the disputed region of Kashmir. All the accused are reportedly living in Pakistan.

Pakistan banned JeM in 2002, a year after it was blamed for an attack on the Indian parliament that took the two neighbours to the brink of war.

It also arrested the group's leader in the wake of the 2008 Mumbai attacks, but he was later released.

NIA investigators said Asghar had posted a video message claiming responsiblity for the Pathankot attack, which came days after Indian Prime Minister Narendra Modi made a surprise Christmas Day visit to Pakistan to meet his counterpart.

The NIA said the attackers stole a taxi after crossing into India, killing the driver, before hijacking a police officer's car to reach the air base.

It said four Pakistani militants had been killed in the January 2 attack. Initial reports had put the number of attackers at six.