President Trump on Wednesday announced a travel ban on citizens of 12 countries, mostly in Africa and the Middle East, aiming to prevent or severely limit their entry to the United States.

The policy is similar to a ban that Mr. Trump put into effect in 2017, during his first term. It is the latest move in the administration’s crackdown on immigration, and was set to take effect on Monday.

What countries are on the list?

A full travel ban will affect citizens of the following countries:

• Afghanistan

• Chad

• Republic of Congo

• Equatorial Guinea

• Eritrea

• Haiti

• Iran

• Libya

• Myanmar

• Somalia

• Sudan

• Yemen

Partial restrictions will apply to citizens of other countries, meaning they cannot come to the country permanently or apply for certain visas. They are:

• Burundi

• Cuba

• Laos

• Sierra Leone

• Togo

• Turkmenistan

• Venezuela

What is the administration saying?

Mr. Trump, who announced the ban in a proclamation, said it was intended to protect “the national security and national interest of the United States and its people.” It is his latest effort to further restrict immigration since returning to office in January, coming after his administration blocked asylum seekers at the Southern border, barred international students from Harvard University and ordered immigration raids across the country.

Cabinet officials in April had identified a list of countries where vetting and screening information surrounding visa applicants was “deficient” enough to warrant a full or partial suspension, the action said.