Indian police said on Tuesday they have arrested four forest guards on suspicion of helping to cover up rhino poaching in a national park visited by Britain's Prince William and his wife last month.

The arrests relate to an incident last year, but came as wildlife officials said a gang of poachers armed with automatic weapons had killed another rhino on Monday in the Kaziranga National Park in the northeastern state of Assam.

The four arrested guards were suspected of covering up the killing of a rhino last November, Kaziranga wildlife warden Suvasish Das told AFP by telephone.

Police said they had failed to report the killing and had buried the carcass with its horn removed.

"Based on specific information, we managed to recover the carcass on Monday after digging up the site," said a senior police officer, speaking on condition of anonymity.

"This led to the arrest of seven people including the four forest guards."

The other three are locals living around the park, home to the world's largest concentration of rare one-horned rhinoceros.

At least seven rhinos have been killed there so far this year. Across the state, the figure is 13.

Images of the park, a major tourist attraction, shot around the world last month when Britain's Duke and Duchess of Cambridge went on safari and fed rhino and elephant calves there during a tour of India and neighbouring Bhutan.

The state's forests minister Atuwa Munda said he had ordered an investigation into the alleged collusion of forest guards in poaching.

"We will take stern action against anyone found (to be involved) in incidents of rhino poaching," he said.

Kaziranga has fought a sustained battle against rhino poachers who kill the animals for their horns, which fetch huge prices in China and Vietnam where they are deemed to have aphrodisiac and cancer-curing qualities.

A recent census estimated there were 2,500 one-horned rhinos in the park out of a global population of around 3,300.