In an aggressive attempt to prevent new coronavirus infections, New York City will begin sending law enforcement officers to check for travelers at Penn Station as well as major bridge and tunnel crossings who are driving in from states with high levels of confirmed cases, Mayor Bill de Blasio said on Wednesday.

"Travelers coming in from those states will be given information about the quarantine and they will be reminded that is it required, not optional," de Blasio said. "They'll be reminded that failure to quarantine is a violation of state law and it comes with serious penalties."

One in five confirmed infections in New York City have been linked to an individual from outside the state, according to Dr. Ted Long, the head of the city's Test and Trace Corps.

The mayor announced the new policy during his morning press briefing, calling it a necessary step as New York increasingly becomes an island among a sea of states with high infection rates. The state's positivity rate has been below 1.5% for more than a month. New York City has been below 3% since June 10, a period of 8 weeks.

Meanwhile, infection levels across the country have been rising. A total of 35 states and U.S. territories, essentially most of the country, currently meets the state's quarantine criteria for a 14-day home quarantine. Rhode Island was added to the list on Tuesday.

Those who fail to quarantine are subject to a $10,000 fine. Governor Andrew Cuomo last month announced that Port Authority police as well as state health officials would start questioning arriving passengers at airports to make sure they filled out the New York State Department of Health travel form, which asks for itinerary and personal contact information. Refusal to fill out the form carries a $2,000 penalty.

On Wednesday, the mayor said that all air travelers would now be required to fill out the state health form when they purchase their ticket online.

The initiative will be led by the Department of Finance's Sheriff's office and is set to begin Thursday at Penn Station, where a team from the mayor's office will educate travelers about the state's quarantine rules. The plans call for about 20 officers in visible locations but the number could fluctuate over time, said Sheriff Joseph Fucito.

Fucito said that officers would perform random checks on vehicles, but would also look at their license plates. He said the stops should not result in traffic being backed up at the bridge and tunnel crossings.

Similar to when Cuomo announced the state's quarantine directive, de Blasio was asked how city officials could enforce the quarantine when tens of thousands of people enter the city each day.

The mayor argued that setting up checkpoints would send a "powerful message" to travelers.

"We’ve got to make it clear to people that there are consequences," he said.