GUWAHATI, India, Feb 25 (Reuters) - A massive manhunt is underway in the dense jungles of India's troubled northeastern state of Manipur after 16 policemen were killed in a militant ambush, a top military commander said on Sunday.

In the deadliest attack in the state for at least two years, more than 30 heavily armed militants threw grenades and fired from hilltops at a police patrol vehicle in Tamenglong district, about 85 km (50 miles) northwest of the state capital, Imphal.

The attack on Saturday came a day after provincial elections were held in Manipur, which borders Myanmar.

Twelve policemen died on the spot and four in hospital. Five others were critically wounded. No militant group has claimed responsibility for the attack.

"The militants meticulously planned their ambush in an isolated place from where they could easily escape," the senior officer supervising the search said, requesting anonymity.

He said soldiers on foot, backed by helicopters, were combing the jungles where the guerrillas are believed to hiding.

"They will not be allowed to go off so easily after killing so many of our boys," he added.

More than 20,000 people have been killed in decades of insurgency in Manipur, triggered in the 1960s after Manipuris complained their province was being neglected by the far-away federal government in New Delhi.

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