A huge religious gathering in northern India could fuel a surge in COVID-19 cases, the government warned Sunday as it called for an increase in testing and other health protocols.

The annual Kumbh Mela festival usually attracts millions of devout Hindus over three months, but was shortened to 30 days as India -- the world's third-most infected nation with nearly 11.6 million coronavirus cases -- grapples with the pandemic.

Authorities had already warned about a recent uptick in infections nationally, with some states imposing restrictions on movement and activities.

Virus fears have not stopped large crowds of pilgrims -- mostly maskless and with no social distancing -- from attending the gathering, with more than three million pilgrims taking part one day earlier this month.

Up to 40 pilgrims and locals were testing positive every day at the festival in Haridwar in Uttarakhand state, the health ministry said.

"This positivity rate has the potential to rapidly turning to an upsurge in cases, given the expected large footfall during Kumbh," it added.

More than 12 Indian states have shown a jump in COVID-19 cases in the past few weeks, the ministry said, warning that some pilgrims could be from these badly affected regions.

Some 50,000 rapid antigen tests and 5,000 RT-PCR tests are being carried out every day at the festival. But the ministry said more RT-PCR tests needed to be done -- particularly in areas where high transmission appears likely.

Crowd sizes are expected to swell during three upcoming holy bathing days.

The government also called on Uttarakhand state authorities to ensure "strict adherence to COVID-appropriate behaviour".

There were 43,846 fresh cases reported in the past 24 hours, the health ministry said Sunday, the highest single-day rise this year.

Infections fell to below 9,000 new cases a day in early February, from a peak of almost 100,000 in September.