(Updates with separate attack)

BHUBANESWAR, India, June 30 (Reuters) - Police and soldiers scoured a remote ravine in India's east on Monday looking for dozens of elite anti-insurgency officers feared dead after Maoist rebels attacked and sank their boat in a reservoir.

Police said 29 officers, some with gunshot wounds, had survived, but 37 others were still missing after the rebels fired from hilltops at their boat passing through a narrow gorge in Orissa state's Malkangiri district on Sunday.

Although police said the well-trained officers would be able to survive, a top official of the local administration said "30-40 people" could have died in the attack.

The anti-insurgency unit was looking for rebels in their jungle stronghold when they were attacked, police said, adding that many of the officers jumped into the water.

"The search operation is on, both in the water and in the nearby forest," Sanjib Patnaik, a top police officer, said.

"Military and police are using helicopters, but they did not spot any dead bodies yet."

But Nitin Bhanudas Jawale, Malkangiri's chief administrator, told Reuters chances of survival in the hostile terrain were low.

Sunday's attack was the biggest since rebels killed more than 50 policemen in neighbouring Chhattisgarh state last year.

Indian media, quoting eyewitnesses, said the Maoists used grenades and rockets to ambush the vessel, a claim that police denied.

Rescue workers said some policemen could have been trapped in the boat and drowned.

In a separate attack, three policemen were killed and four injured in a Maoist landmine attack in the jungle in the eastern state of Jharkhand on Monday, police said.

Maoist rebels regularly kill police and attack government establishments and factories in a large swathe of eastern and central India, particularly in the countryside.

They say they are fighting for the rights of the poor and landless, an insurgency Prime Minister Manmohan Singh has described as the single biggest threat to India's internal security. It has killed thousands of people in the past four decades. (Reporting by Jatindra Dash; Editing by Krittivas Mukherjee and Alex Richardson)

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