More than a dozen known or suspected terrorists are trying to get into the U.S. on any given day, Homeland Security Secretary Kirstjen Nielsen told Fox News, citing new internal department data.

"People think of border security in very different ways but to me it’s very simple: border security is national security," Nielsen said. "What we see at the Department of Homeland Security, we see 15 terrorists either planning to travel or actually traveling to the United States each day, known and suspected terrorists. So that means they are coming through our legal land, port and air but they could be coming across that (southwest) border."

In New York City for a series of recent closed-door threat and intelligence meetings, Nielsen spoke exclusively with Fox News at Ground Zero -- not far from the October 2017 attack where an ISIS-inspired terrorist used a truck to kill eight people along the West Side Highway.

"Using the Internet, terrorist groups are encouraging followers to bring your own weapon, use a car, use a truck, use a knife, something you can easily pick up, and go and commit chaos and murder," Nielsen explained.

The Chief of Counterterrorism at the New York City Police Department, James R. Waters, said personal relationships matter. "Being able to pick up the phone and speak to the secretary herself or her immediate staff and discuss an issue or a problem to protect the city is invaluable."

Waters' intelligence teams track events worldwide, including the attempted poisoning in the U.K. targeting a Russian military officer-turned-spy and his daughter last month. "We study attacks around the world and if something can be done over there, it can be done over here," Waters explained to Fox News. "So that has been the secret to some of our success in thwarting plots... we follow the threat. We follow the attacks that go on. We travel to those parts of the world and talk one-to-one with the law enforcement and their governments."

Waters said that since 9/11, his teams have thwarted "more than 20 plots. Bona fide plots" targeting the New York City area.

With the Supreme Court now weighing the third version of the travel ban affecting a small group of mostly Muslim countries, Nielsen said the Trump administration remains committed to knowing who's entering the U.S., and whether their home nations are high-risk.

"What we’ve done as a country is, we’ve said we owe it to our citizens to know who is traveling here and if they intend to do us harm."

Asked if the administration was dissuaded from the travel ban given the setbacks, Nielsen said, "I’m not. I think it’s a requirement, I think that’s what Americans expect their government to do, is to ensure that people traveling here do not intend to do them harm."

Nielsen said that in her first four months on the job, her department has run into roadblocks with Congress. On funding for the border wall, she said lawmakers "all say to me, 'we’re in favor of border security, border security is our sovereignty, border security is the most important thing a country can do,' and yet when we try to work with them on legislation it falls apart. So, I really hope that they take it seriously. I’m willing to work with anyone in good faith who will work with me but it’s very high on the president’s mind to get this done as soon as possible."

Speaking about the recent deployment of National Guard troops to the Mexican border, "I don’t know why it was controversial. We’ve done this twice before to much success." Asked if that was a measure of the politically charged environment, Nielsen said, "I think so."

Nielsen pushed back against critics who said she got the top job because she worked for the administration's first DHS chief, John Kelly, now the White House chief of staff.

"I worked at the crossroads of policy and operations," Nielsen told Fox News. "I kind of bring all that experience together because homeland security is a team sport, and I've played almost every role, every player, every team. So from that perspective I think I bring that uniqueness so that policy is never made in a vacuum and operations are never made in such a way as to violate a policy."

Almost every homeland security secretary makes the pilgrimage to Engine 10, ladder 10 near the World Trade Center site.

"It’s moving, a reaffirmation of why we do this, the importance of the mission, the sacrifice of the first responders who on a good day put their lives on the line to protect our fellow citizens."