India has reported a new "Delta plus" coronavirus variant of concern, officials have said.
Sixteen cases of the variant, a mutation of B.1.617.2, were found in the state of Maharashtra on Tuesday, federal health secretary Rajesh Bhushan told a news conference.
Delta plus is thought to be more transmissible than the original Delta variant, according to state officials who are increasing testing and local vaccination drives.
Public Health England has said the Delta variant is up to 60% more contagious than the Alpha (Kent) one that emerged in December last year.
Currently in the UK, Delta makes up more than 99% of reported COVID-19 cases.
A Nepalese mutation of the Delta variant was reported to be behind Portugal being moved from the green to amber travel lists last month.
Watch: Algarve travellers hit by rule changes
Experts have repeatedly said that the virus will mutate constantly over time, with some variants posing a risk, as they could escape the immune system.
PHE has said two doses of both the Oxford-AstraZeneca and Pfizer-BioNTech vaccines are effective against the Delta variant - but more data would be needed on a further mutation.
On Monday, India vaccinated 8.6 million of its 950 million adults as it began offering free doses to all over-18s.
Since May, India has vaccinated fewer than three million people a day - far below the 10 million officials say are needed to prevent future surges.
Only 5.5% of the adult population is vaccinated so far, despite India being the world's biggest vaccine producer.
Case numbers are the lowest they have been in more than three months - but experts say vaccinations need to be stepped up to mitigate against new variants.
A huge spike in infections of the Delta variant crippled India in April and May, with hospitals running out of oxygen and mass graves unable to contain the numbers of dead.
Watch: Doctors scramble for oxygen supplies as India's coronavirus crisis worsens