Bengal registered as many as 1,802 COVID-19 cases in last five days, with the tally rising to 7,303 in the state in an alarming spike in 5,501 infections recorded in two months up to May 31, when the government called for easing lockdown curbs.
Senior doctors at state-run hospitals said "unplanned relaxation of lockdown norms" and return of migrants in hordes were to blame for the surge in numbers.
Some of the doctors also said the lockdown did not yield the expected results, as migrant movement was allowed amid the spike in cases, and curbs were eased abruptly.
"The rise should be attributed not just to return of people from other states, but also the unnecessary rush to ease the lockdown norms. Health experts had been warning against such moves, as the outbreak might just get out of control. Total shutdown should have continued at least for a few more weeks," a senior doctor at one of the dedicated COVID-19 hospitals in the city said.
According to the doctor, who did not wish to be named, people had been travelling from one place to another with no regard for social distancing norms, since transport vehicles resumed services in the state.
In Kolkata alone, a total of 2,589 coronavirus cases were registered till Friday, up from 1,000 in mid-May - a rise of over 1,500 cases in less than a month.
Similar were the scenarios in adjacent districts of North 24 Parganas, Howrah, Hooghly and South 24 Parganas.
North Bengal, which was largely unaffected by the pandemic till May, have at least 698 cases at present.
Both Cooch Behar and Alipurduar were green districts until May 27. Several returnees from other states, however, tested positive for the disease in the two districts in the days that followed.
A senior doctor at Beliaghata ID hospital, another facility earmarked for COVID-19 patients, said many of those entering the state are asymptomatic, and stand the chance of shedding virus during travel.
"Some test positive, but there are many who do not at first go. Their travel and contact history are not ascertained and they are allowed to go home... These people come in contact with several others... And you can imagine the risk factor involved.
"If you ask me, I will say that the lockdown should be extended for at least another fortnight," she said.
The government should have given a "clear list of dos and don'ts" before the lockdown was relaxed, the doctor said.
"In the first place, I don't think the lockdown was properly implemented. Had that been done, the results would have been different. Look how South Korea has done it.
"Nobody, it seems, realises what's in store for us under the current circumstances," she added.