New Delhi: Indian police on Monday killed at least 21 rebels in a shoot-out in eastern India, a local officer said, the deadliest such incident this year in a long-running Maoist insurgency.

Police said they ambushed a meeting of 30 to 40 Maoists in a forest near the border of Odisha and Andhra Pradesh states, triggering a gun battle.

“Now 21 bodies of the Maoists have been recovered. The search operation is still on,” sub-inspector C.K. Dharua told journalists by phone from Malkangiri district in Odisha where the attack occurred.

Dharua had earlier confirmed 18 deaths but warned the toll could rise as “there was a large number of people at the meeting”.

A top Maoist leader and his son were suspected to be among those killed, the Press Trust of India news agency said, citing unnamed police.

Two police officers were also injured in the shoot-out that last about an hour, according to local media,

Weapons including four AK-47s and three self-loading rifles were recovered from site, some 640 kilometres from the state capital Bhubaneswar, Dharua said.

India’s Maoist insurgency began in the 1960s, inspired by Chinese revolutionary leader Mao Zedong, and has cost thousands of lives.

The rebels, described by former prime minister Manmohan Singh as India’s most serious internal security threat, say they are fighting authorities for land, jobs and other rights for poor tribal groups.

Monday’s gun battle comes after 10 paramilitary commandos in eastern Bihar state were killed in July after suspected Maoist rebels ambushed their convoy and set off a series of homemade bombs.

In March, suspected rebels triggered a powerful landmine blast in central Chhattisgarh, killing seven policemen.

The South Asia Terrorism Portal (SATP) website, which tracks separatist trends, showed Monday’s attack as the deadliest Maoist incident this year. The insurgency has claimed more than 7,000 lives between 2005 and 2016, according to SATP.

Maoist sympathiser and author Varavara Rao cast doubt on the police version of Monday’s events, saying it was likely officers simply ambushed the rebels, with no little time for them to return fire.

“With [the] information [that] you have, [you have] gone and hurled grenades there and then they have fired in self-defence,” Rao told the NDTV network.

The rebels operate in at least 20 Indian states but are most active in the forested and resource-rich areas of Chhattisgarh, Odisha, Bihar, Jharkhand and Maharashtra.

They draw recruits from tribal communities whose members are often desperately poor and living in underdeveloped areas neglected by successive governments.

Government critics say attempts to end the revolt through tough security offensives are doomed to fail, and the real solution is better governance and development of the region.

Prime Minister Narendra Modi has been seeking to stem the insurgency by earmarking development funds for revolt-hit areas and improving policing.

Last year, Modi urged Maoists to put down their guns and take up ploughs, saying “violence has no future”.

The remote forests of Malkangiri district are a major transit point for rebels because it borders Maoist-strongholds in Chhattisgarh and Andhra Pradesh, the Hindu newspaper said.