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WASHINGTON (AP) - Since Civil War times, the U.S. has tried to prevent its citizens at various points from traveling to certain countries. Under the Passport Act, the secretary of state can ban U.S. passports from being used to enter countries at war with the United States or that pose significant security threats to Americans.

Sometimes, political considerations are taken into account. In 1952, during the Cold War, all U.S. passports were amended with a stamp indicating they weren't valid for travel to China, the Soviet Union and other Warsaw Pact nations without a State Department exemption. The restriction for China ended in 1971. Others were phased out over time.

Should Secretary of State Rex Tillerson use his authority for North Korea, it would be the first time a "geographic travel restriction" has been in place in 13 years.

Some recent bans and when U.S. passports were deemed invalid for travel:

-Cuba: Starting in 1963, after the U.S. broke diplomatic ties with the communist island. It lasted until the opening of the U.S. Interests Section in Havana in 1977.

-Iran: After the 1979 Islamic Revolution and ensuing hostage crisis at the U.S. Embassy in Tehran. The ban was in place from 1980-1981.

-Lebanon: Lasting from 1987-1997 during the Lebanese civil war and a period in which numerous Americans and other Westerners were abducted.

-Iraq: Between 1991 and former Iraqi dictator Saddam Hussein's overthrow in the U.S.-led invasion in 2003.

-Libya: From 1981-2004. The ban was imposed by the Reagan administration after the Gulf of Sidra incident in which subway runner map two Libyan fighters fired on U.S. aircraft in a Mediterranean Sea naval exercise. It was not until after former Libyan leader Moammar Gadhafi renounced terrorism and weapons of mass destruction in 2003 that it was rescinded.