Police said more than 100 soldiers and armed police had been sent to investigate a tip-off on the whereabouts of a rebel camp in Chhattisgarh state on Monday. Twenty-three bodies have been found and more than a dozen more were missing, feared dead.

"It looks like it was a set-up," a senior Home Ministry official, who asked not to be named, told AFP -- referring to what appeared to be a well-planned ambush by the increasingly active ultra-leftist rebels.

Officials said the corpses of 23 soldiers and police had been found in the Maraeguda jungles, around 500 kilometres (310 miles) from the state capital Raipur.

"Some 25 Maoists also seemed to have been killed in the encounter," senior police officer RK Vij said after visiting the scene of the battle, although he added that no rebel bodies have yet been found.

Officials said the police and soldiers may have been outnumbered by five to one.

Maoist rebels, who launched their armed campaign in 1967 and say they are fighting for the rights of landless farmers, hold sway in 10 of Chhattisgarh's 16 districts.

They also operate in 14 of India's 29 states, with their area of activity spanning from the border with Nepal down to Andhra Pradesh state in the south.

Prime Minister Manmohan Singh has described them as the most serious threat to national security.

"State commandos backed by commandos from Andhra Pradesh (state) are also rushing to Dantewada," said Abhishekh Dayal, a spokesman for the paramilitary Central Reserve Police Force (CRPF).

Two combat helicopters have been sent to search for the missing troops in heavily-wooded area, with sporadic skirmishes also reported.

In March, 55 policemen were killed in the worst-ever Maoist attack in Chhattisgarh.

That attack followed the murder of high-profile federal MP Sunil Mahato in neighbouring Jharkhand state -- the first assassination of a national-level politician by the outlawed Maoists.

Security analyst PV Ramana said increased attacks by the rebels were a "tactical counter-offensive campaign to force the government and security personnel onto the back foot."

"There are even more attacks to come," predicted Ramana, who works at the Observer Research Foundation think-tank in New Delhi.

B Raman, a former intelligence official said the Maoist movement had gained "a lot of public support as the Maoists are raising economic issues and causes -- for instance, exploitation by landlords, taking away of forest lands."

Tens of thousands of people have been displaced in troubled Chhattisgarh by the conflict and are now living in tent shelters as Indian counter-insurgency forces operate in the countryside where the guerrillas hold sway.