India has said a "double mutant variant" of the coronavirus first discovered there in March may be "linked" to a deadly second wave there.

Samples containing the mutant - or B.1.617 variant - have been found in several states with high case numbers.

An official with the National Centre for Disease Control said however, that they had still been unable to "fully" establish a correlation.

A double mutant is when two mutations come together in the same virus.

Meanwhile, India reported a record 412,000 cases in the space of 24 hours on Wednesday, and 3,980 deaths.

The government's top scientific adviser also warned a third wave is inevitable.

Speaking at a health ministry news briefing, K VijayRaghavan admitted that experts had not anticipated the "ferocity" of the surge in cases.

"Phase three is inevitable, given the high levels of circulating virus," he said at a news briefing. "But it is not clear on what timescale this phase three will occur... We should prepare for new waves."

The current surge of the virus has already overwhelmed the healthcare system with hospital beds, oxygen and even crematorium space in short supply.

Several states are under localised lockdowns and curfews, but the government is reluctant to impose a national lockdown, for fear of the impact on the economy.

Where has the 'double mutant' variant been found?

Out of roughly 13,000 samples sequenced, more than 3,500 were found to be variants of concern across eight states.

The B.1.617 variant was found in several states reporting a surge including Maharashtra, Karnataka, West Bengal, Gujarat and Chhattisgarh.

For more than a month, Delhi insisted that the B.1.617 variant had no link to the current surge.

Officials have also denied that the rise in cases is linked to the British, South African and Brazilian mutations.

Virologist Dr Shahid Jameel told the BBC's Vikas Pandey earlier that India started seriously looking at mutations fairly late, with sequencing efforts only "properly started" in mid-February 2021.

India is sequencing just over 1% of all samples at the moment. "In comparison, the UK was sequencing at 5-6% at the peak of the pandemic. But you can't build such capacity overnight," he said.

And although the central government now says there is a correlation, it added the link is not "fully established".

"Its epidemiological and clinical correlation is not fully established…without the correlation, we cannot establish direct linkage to any surge. However, we have advised states to strengthen public health response — increase testing, quick isolation, prevent crowds, vaccination," Sujeet Singh, National Centre for Disease Control said.