India overtook Brazil on Monday to become the second-worst affected country in terms of coronavirus cases behind the United States, according to an AFP tally.

The South Asian nation has recorded 4.20 million infections since the pandemic began, health ministry data showed, compared with 4.12 million in Brazil and 6.25 million in the US.

India has also recorded 71,642 deaths, fewer than the 126,203 in Brazil and 188,540 in the US.

Many experts, however, say it is not testing enough people and not properly recording many deaths, meaning the real numbers may be much higher.

Since August the country of 1.3 billion people, home to some of the world's most densely populated cities, has been reporting the highest single-day rises in the world. On Monday it reported an increase of more than 90,000 cases.

Its caseload moved past four million on Saturday, only 13 days after hitting three million.

Virologist Shahid Jameel, who heads the Wellcome Trust/DBT India Alliance, said the key factor to watch is the growth rate in infections, which he called "quite alarming".

"Over the past two weeks, the... average has moved from about 65,000 cases per day to about 83,000 cases per day, that is about a 27-percent increase over two weeks or about two percent per day," Jameel told AFP.

India has been testing more than 10 million people per day on average, with plans to ramp it up further.

The Indian Council of Medical Research, the scientific agency leading the government's response, on Friday revised the testing criteria, allowing anyone to undergo a test without a doctor's letter.

Jameel said the move was overdue. "This will uncover more asymptomatic people, who are the real source of this expansion in India. There should also be more testing in rural districts and villages, since over two-thirds of the cases are coming from there," he said.

Despite warnings that it could eventually overtake the US to become the world's most-infected country, India has been steadily reopening its pandemic-battered economy.

Economic output contracted a historic 23.9 percent between April and June.

On Monday, metro trains in major cities, including the capital New Delhi -- one of the most badly hit cities along with financial hub Mumbai -- reopened after a hiatus of nearly six months.

Early morning images showed masked commuters sitting in nearly empty coaches and flashing victory signs to journalists.

Passengers can only sit on alternate seats, and after undergoing thermal screening.

Delhi, a city teeming with 20 million people, recorded 3,256 new infections on Sunday -- its highest single day spike in 73 days. It was also the first time cases crossed the 3,000 mark during this period.