Homeland Security Secretary Kirstjen Nielsen said Monday that her department may have been founded to combat terrorism, but its mission is shifting to also confront emerging online threats.

China, Iran and other countries are mimicking the approach that Russia used to interfere in the U.S. presidential election in 2016 and continues to use in an attempt to influence campaigns on social media, she said. Under threat are Americans' devices and networks.

"It's not just U.S. troops and government agents on the front lines anymore," Nielsen said. "It's U.S. companies. It's our schools and gathering places. It's ordinary Americans."

Devices and networks are "mercilessly" targeted, she said. Those responsible are "compromising, co-opting, and controlling them."

Nielsen was speaking about the priorities of a sprawling department created after the Sept. 11 attacks. It handles counterterrorism, election security and cybersecurity, natural disaster responses and border security — President Donald Trump's signature domestic issue.

The president on Friday issued his first veto , to secure money for a U.S.-Mexico border wall. Nielsen did not specifically mention that fight, but made clear that she sees a humanitarian and security crisis at the border because of an increasing number of Central American families crossing into the U.S. to seek asylum.

While the overall number of migrants coming into the U.S. is down from a high of 1.6 million in 2000, the number of families crossing the U.S.-Mexico border has reached record highs. The system is at a breaking point, she said.

Nielsen said the department has introduced tougher screening systems at airports and is working with the State Department to notify other countries of stricter information-sharing requirements. She said the countries that work with the U.S. will make the world safer, and those that do not "will face consequences."