The worst outbreak of coronavirus in Europe has spurred the U.S. government to issue the highest level of warning Sunday to the two Italian regions hardest hit by the virus.

The State Department updated its travel advisory to its highest level -- Level 4 -- urging Americans, "Do Not Travel" to the Lombard and Veneto regions in northern Italy.

The travel advisory cited quarantines set up in 10 Lombard towns and one in Veneto, with a combined population of 50,000 people, as well as "the level of community transmission of the virus"

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Sunday's message comes after an earlier warning to avoid non-essential travel to all of Italy, where more than 1,694 cases were confirmed through Sunday -- a 50 percent jump from just 24 hours earlier.

"Reconsider travel to Italy due to a recent outbreak of COVID-19," the earlier advisory stated. "There is an ongoing outbreak of COVID-19 caused by a novel (new) coronavirus in Italy. Many cases of COVID-19 have been associated with travel to or from mainland China or close contact with a travel-related case, but sustained community spread has been reported in Italy."

Five more people infected with the virus have died, bringing the deaths in Italy to 34, while 83 have fully recovered.

Lombardy, which includes Italy's financial capital Milan, accounts for just over half of the cases while Veneto and Emilia-Romagna have 18 percent and 20 percent, according to a roundup by the Associated Press.

All three regions have closed schools for the time being. In Veneto and Lombardy, closures also have hit museums, theaters, cinemas, and most public offices, emptying urban centers like Milan, where many companies permitted office workers to telecommute.

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Italian health authorities said the increases were expected since it takes up to two weeks for containment measures to take effect and because Italy has a large number of elderly people. Still, the numbers highlighted the rapid impact the virus is having on Italy, the epicenter of the outbreak in Europe.

"This acceleration was expected, unfortunately," said Giovanni Rezza, director of the infective illness department at the National Health Institute. He said it would be another week or 10 days until the spread of the virus slowed down in the country.

Analysts have warned that the outbreak looks set to shunt Italy’s fragile economy into its fourth recession in 12 years, according to Reuters.

Tourism officials have cited the previous warning covering all of Italy as potentially calamitous to the industry, which represents 13 percent of the gross domestic product in a country famed for its world-class museums, archaeological sites, art cities, and natural beauty.

On Sunday, Delta Air Lines said that following travel guidance from the State Department it will temporarily suspend its daily flight between New York's John F. Kennedy International Airport and Milan Malpensa Airport. Delta’s last flight from JFK to Milan will depart on Monday, March 2, and the last flight from Milan to JFK will depart on Tuesday, March 3.

Service between the two cities will resume starting on May 1 and May 2, respectively, according to the airline.

"The airline’s daily flights between Rome and both JFK and Atlanta continue to operate as scheduled," Delta said.

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More than 5.6 million Americans visit Italy every year, representing 9 percent of foreign tourists and the second-largest national group behind Germans, according to the most recent statistics.

As the coronavirus outbreak worsens in the northern part of the country, the French community church in Rome, St. Louis of the French, closed its doors to the public on Sunday after a priest was infected with the virus.

A sign on the door Sunday noted in French that the church had been closed as a precaution by the French Embassy for both Masses and touristic visits until further notice, according to the AP.

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The Religious Information Service news agency reported the church was closed after a 43-year-old priest who had returned to Paris was hospitalized after being infected by coronavirus.

The service carried a statement by the archbishop of Paris, Michel Aupetit, who said the priest, who had been living in Rome, returned to Paris by car in mid-February and tested positive for the virus on Friday. The priest was in good condition, Aupetit said.

It was the first church in Rome closed by the virus. Churches in much of Veneto and Lombardy have closed their doors under widespread measures aimed at containing the spread of the virus. Televised Masses were available for the faithful.

Fox News' Louis Casiano and The Associated Press contributed to this report.