ATLANTA — All travelers who come into the U.S. from three Ebola-stricken West African nations will now be monitored for three weeks, the latest step by federal officials to keep the disease from spreading into the country.

Starting Monday, anyone traveling from Guinea, Liberia and Sierra Leone will have to report in with health officials daily and take their temperature twice a day.

The measure applies not only to visitors from those countries but also returning American aid workers, federal health employees and journalists. The Centers for Disease Control and Prevention announced the new step Wednesday.

Passengers already get screened and their temperatures checked before they leave West Africa and again when they arrive in the United States.

“We have to keep our guard up,” CDC Director Tom Frieden told reporters on a conference call.

The Obama administration has resisted increasing pressure to turn away visitors from the three countries at the center of the Ebola outbreak, especially after a Liberian visitor to Dallas died of the infectious disease. Instead, passenger screening was put in place at five key U.S. airports. That was tightened Tuesday to funnel everyone coming from those countries through those airports so all are checked.

The monitoring program will start in six states — New York, Pennsylvania, Maryland, Virginia, New Jersey and Georgia — the destination for the bulk of the travelers from the outbreak region. It will later extend to other states.

Each passenger will be required to provide contact information for themselves as well as a friend or relative. They will be instructed to check for a fever twice a day and report their temperature and any symptoms to health officials daily for 21 days.

How the checks are done — in person, by phone or Skype — will be decided by the states, Frieden said. If a traveler does not report in, public health officials can track them down. How far they can go to get them to cooperate is up to those officials, CDC officials said.

They will also receive “CARE” kits — the name stands for Check and Report Ebola. The kits include a thermometer and instructions on what to do if symptoms occur. Also included is a card to present to health care providers if they seek care.

Earlier this year, roughly 150 travelers to the U.S. each day were from the three countries. But it appears there are far fewer now — there are no direct flights and flights to the area have been curtailed.