Officials in Lombardy, which includes Italy's financial capital Milan, said cases had risen overnight to 259 from 240 on Tuesday, with four children, including a 4-year-old girl, infected in the first such cases in the country.
In neighboring Veneto, the number of people confirmed to have caught the flu-like virus was 58, an increase of 13 on the previous tally given on Tuesday.
The death toll from the contagion, which came to light on Friday, remained unchanged at 11. All those who have died so far have been elderly and most had underlying health problems.
After first emerging in Italy in Lombardy and Veneto, the country's economic powerhouse, the illness has now spread to seven other regions, including Sicily in the far south, with the total number of cases nationwide climbing above 350.
Italians or people who had recently visited the north of the country have tested positive in Spain, Austria, Switzerland, Croatia and France since the weekend, showing how far and fast the illness - which was first identified in China last month - could spread.
In bid to halt the outbreak, authorities have shut schools, universities, museums, cinemas and theaters across much of the north. Many countries have advised their nationals not to visit the north of Italy and hoteliers have reported a wave of cancellations, putting the local tourism industry at threat.
"We should stay calm, there is no reason to be particularly afraid," said Elisabetta Jacona, a Milan resident and doctor.
"The only advice I can give, as a doctor, is telling people who are more at risk, elderly people or people with previous pathologies ... to avoid going out."Analysts have warned that the outbreak could shunt Italy's fragile economy into its fourth recession in 12 years.