India issued an advisory to all states to rigorously test and screen international travellers from South Africa and other "at risk" countries amidst concerns over a new coronavirus variant, after easing some of its travel restrictions earlier this month.

The federal health ministry said reports of mutations in the new variant, identified as B.1.1.529, had "serious public health implications".

"This variant is reported to have a significantly high number of mutations, and thus, has serious public health implications for the country in view of recently relaxed visa restrictions and opening up of international travel," health secretary Rajesh Bhushan said in a letter to states issued late on Thursday.

On Friday, the UK Health Security Agency said the new variant has a spike protein that was dramatically different to the one in the original coronavirus that Covid-19 vaccines are based on. And it could make existing vaccines less effective.

Britain has banned flights from six African countries, and asked returning British travellers from those destinations to quarantine.

India, the world's second-worst affected country by Covid-19, posted the smallest rise in new cases in one-and-a-half years this week, due to rising vaccinations and antibodies in a large section of its population from previous infections.

The country's total cases of the coronavirus reached 34.56 million on Friday. India's daily caseload has halved since September and the country reported 10,549 new cases on Friday.

Earlier this month, India identified 10 countries "at risk" that includes Europe, Britain, China, South Africa, and New Zealand, among others. And has opened its borders to 99 countries overall.

Indian shares tumbled more than 2 percent on Friday, as investors fled risky assets panicking over the impact of the new variant. The losses were in line with steep falls seen in markets across Asia.