GUWAHATI, India (AFP) - An armed gang has killed two rare rhinoceros at a wildlife park in northeast India, officials said on Thursday, adding that drones deployed to stem the rising number of killings have been halted.

Poachers used assault rifles to shoot dead the rhinoceros before gouging out their horns at the park in Assam state on Wednesday, taking the total number slaughtered there this year to 27, officials said.

"Poachers used AK 47 to shot dead the rhinos. We have recovered empty cartridges from the site of the incident," a park ranger said, requesting not to be named.

Assam forest minister Rockybul Hussain said the killings at Kaziranga National Park, a world heritage site, were carried out by "militants", while declining to name the outfit thought responsible.

Mr Hussain also said the state has been forced to halt drone flights over the reserve aimed at safeguarding the one-horned rhinoceros, the first time the country had used aerial technology to protect wildlife.

He said the federal defence ministry has ordered an end to the use of the unmanned aerial vehicle (UAV) for security reasons, a blow in the campaign to protect the rhinos.

"For security reasons we have been asked by the defence ministry to discontinue the UAV. We want the central government to review the decision," the minister told reporters.

He said he would not reveal what the security concerns were.

The government hailed the drones in April this year as a new weapon in the fight against poachers, allowing patrols of previously unreachable areas, from the sky, and giving rangers a safe view of illegal activities on the ground.

The 430-square-km park in Assam is home to the largest concentration of the world's remaining one-horned rhinoceros.

Kaziranga has fought a sustained battle against poachers who kill the animals for their horns, which fetch huge prices in some Asian countries where they are prized for their supposed aphrodisiac and medicinal qualities.

Forest department records show 27 rhinos have been killed in the park this year, up from 21 last year, prompting the Assam government to hand over investigations to India's Central Bureau of Investigation, the equivalent of the FBI in the United Sates.

"The CBI investigations are on and hopefully they are able to finish their probe soon," the minister said.

A 2012 census in the park put the number of one-horned rhinoceros at 2,290 out of a global population of 3,300.

The species fell to near extinction in the early 1990s and is currently listed as "vulnerable" by the International Union for Conservation of Nature.