At least 114 people have died in the past three weeks after infections following monsoon rains have led to a fever outbreak in Uttar Pradesh state in northern India, health officials have said.

State health minister Jai Pratap Singh said that most cases were caused by dengue, a seasonal viral infection spread by mosquitoes, followed by leptospirosis, scrub typhus, and malaria.

Leptospirosis and scrub typhus are bacterial infections, while malaria is a mosquito-borne disease caused by a parasite. They typically spike after the rainy season in India.

The minister said the dengue cases were due to a virulent strain called D2, which had been detected in some of the state's districts in a sample survey carried out by a team of the Indian Council of Medical Research.

"This is a different strain as compared to the regular strain of dengue and there is a need to be careful," he said.

Dengue, also called "breakbone fever" because of the severe pain it causes, is not always fatal but severe cases can require hospitalisation.

Prevention efforts targeted at destroying mosquito-breeding sites, like removing rubbish or old tyres and other objects containing standing water, were the best ways to curb the spread of the disease, according to health experts.

Utkarsh Singh, a senior health official in Uttar Pradesh state's Firozabad, said the district's main hospital had recorded more than 1,200 cases of fever linked to the outbreak since August 30, prompting authorities to use hospital facilities reserved for Covid-19 cases. He said most of the patients were children.

"We do not have enough beds to keep them. We are forced to accommodate three to four children on one bed," the health official said.

The health system in Lucknow, the state capital, was also buckling as a surge in cases had put pressure on hospitals there.

The city had so far reported more than 1,500 cases of fever linked to the outbreak after authorities started conducting door-to-door surveillance last week.

"The viral fever has spread like a wildfire across the state," one not-for-profit health care organisation said.

Uttar Pradesh, India's most populous state, often figures disproportionately among the post-monsoon infection fatalities in the country because of its weak health system.

Thousands of people suffer from dengue, encephalitis, malaria, typhoid, and other mosquito-borne diseases each year during the summer monsoon.