More than 500 rebels stormed Nayagarh town, about 90km from Bhubaneswar, the capital of Orissa state, on Friday night and burned down the three police stations.

Prime Minister Manmohan Singh last year described the Maoist threat as the 'single biggest internal security challenge ever faced by our country'.

Orissa police said 13 policemen and two civilians were killed in the attack, while three more policemen were killed in search operations yesterday.

The attackers also looted two armouries and took away 1,100 weapons, including light machine guns, rifles, pistols and a large quantity of ammunition.

Orissa has sealed its borders to prevent the rebels from escaping, while all neighbouring states have sounded an alert.

Police officials said the rebels were in control of Nayagarh town for more than four hours and left in a hijacked passenger bus after loading it with the looted weapons. They met little resistance.

While police put the number of Maoists who took part in the attack at over 100, Orissa Chief Minister Naveen Patnaik, who visited Nayagarh on Saturday, said they had numbered about 500.

'Intensive combing operations are going on. An additional force of 600 police personnel has been sent to Nayagarh,' Mr Patnaik told the state legislature after the visit.

He claimed that about 60 security personnel had tried to repulse the attack for more than two hours, but they had been outnumbered by the militants.

The rebels included a large number of women cadres and the attack was 'meticulously planned', senior police officials in the area said.

The federal government has taken serious note of the attack, believed to be the biggest ever in Orissa, and sent a team of senior security and intelligence officials to the state to coordinate the operation to track down the Maoists.

At a press conference last evening, Orissa Home Secretary Tarun Kanti Mishra said troops had killed at least 20 Maoist guerillas in retaliatory attacks.

According to official statistics, the militants, who launched an armed movement in 1967 for the rights of peasants and landless labourers, are active in 16 of India's 33 states. They carried out 842 attacks last year, killing 138 police personnel.

pjay@sph.com.sg