India has said it has found around 40 cases of the Delta coronavirus variant carrying a mutation that appears to make it more transmissible, and advised states to increase testing.
The variant, identified locally as "Delta plus", has been found in three states of Maharashtra, Kerala and Madhya Pradesh, with "no significant increase in prevalence".
The earliest case in India is from a sample taken on 5 April.
The ministry of health said Delta plus showed increased transmissibility and advised states to increase testing.
Some scientists worry that the mutation, coupled with other existing features of the Delta variant, could make it more transmissible.
"The mutation K417N has been of interest as it is present in the Beta variant (B.1.351 lineage), which was reported to have immune evasion property," India's health ministry said in a statement.
On Monday, India vaccinated a record 8.6 million people as it began offering free shots to all adults, but experts doubted it could maintain that pace.
"This is clearly not sustainable," Chandrakant Lahariya, an expert in public policy and health systems, told Reuters.
"With such one-day drives, many states have consumed most of their current vaccine stocks, which will affect the vaccination in days to follow."
With the currently projected vaccine supply for the next few months, the maximum daily achievable rate is 4 to 5 million doses, Mr Lahariya added.
The effort has so far covered about 5.5% of the 950m eligible, even though India is the world's largest vaccine producer.
A devastating second wave during April and May overwhelmed health services, killing hundreds of thousands. Images of funeral pyres blazing in car parks raised questions over the chaotic vaccine roll-out.
Since May, vaccinations have averaged fewer than 3m doses a day, far less than the 10m health officials sayare crucial to protect the millions vulnerable to new surges.
Although new infections in India have dropped to their lowest in more than three months, experts say vaccinations should be stepped up because of the transmissibility of new variants.
Over the past 24 hours India reported 42,640 new infections, the lowest since March 23, and 1,167 deaths.
Infections now stand at 29.98m, with a death toll of 389,302, health ministry data showed.
The British government and Public Health England have said they are monitoring the Delta plus variant.
Boris Johnson's official spokesman said PHE had already put extra measures in place where the variant had been detected including "enhanced contact tracing, testing, and isolation".
He said 41 cases of the variant had been identified in the UK.
Additional reporting PA