Amid the Trump administration's ongoing crackdown on illegal immigration, the nation's immigration service is warning immigrants to carry their green card or visa at all times.

U.S. Citizenship and Immigration Services posted the reminder July 23 on social media: "Always carry your alien registration documentation. Not having these when stopped by federal law enforcement can lead to a misdemeanor and fines."

Here's what immigrants – and American citizens – need to know.

The law requiring lawful immigrants and foreign visitors to carry their immigration documents has been on the books for decades, dating to the 1950s.

The Immigration and Nationality Act states: "Every alien, eighteen years of age and over, shall at all times carry with him and have in his personal possession any certificate of alien registration or alien registration receipt card issued to him."