Ten war zones and failed states are ranked under Level Four, with the advice "do not travel", namely Afghanistan, Central African Republic, Iran, Iraq, Libya, Mali, Somalia, South Sudan, Syria and Yemen.
North Korea is also under Level Four, with the additional restriction that US law prohibits American travellers from using their passports there, effectively banning visits.
But some of the other country ratings may raise eyebrows - or international anger - even if the State Department says it is only presenting existing advice in a new format.
Officials insisted that the change was to make advice clearer to US travellers, but the travel warning system has long been controversial and often offends foreign capitals.
"These are not political documents. These are simply based on our assessment of the security situation," said senior consular official Michelle Bernier-Toth.
Major European allies such as Britain, France and Germany are placed under Level Two, meaning travellers should "exercise increased caution", while authoritarian Uzbekistan is under Level One, meaning one should "exercise usual precautions", the lowest advisory level for safety and security risk.
Singapore, Malaysia, South Korea and Japan all fall under Level One.
On Tuesday, US senators heard State Department officials say that someone known to the Cuban government has a mystery weapon that is used to cause brain trauma to Americans in Havana.
These are not political documents. These are simply based on our assessment of the security situation.
SENIOR UNITED STATES CONSULAR OFFICIAL MICHELLE BERNIER-TOTH
But Cuba is rated only Level Three, meaning one should "reconsider travel".
Some countries have complained in the past that the travel warnings exaggerate dangers and damage tourism, or suspect that they have been subjected to a US diplomatic rebuke.
But each warning is accompanied by a country page on the travel.state.gov website, explaining what specific threats have been identified and why the advice has been given.
Western European capitals, for example, while prosperous and politically stable, have seen recent attacks by Islamist militants in areas popular with tourists.
Mexico, a Level Two country, has complained in the past that US alerts hurt tourism, but the site gives a detailed breakdown of areas to avoid because of drug cartel violence.
The United States itself is not rated in the new system. But as a rough guide, America's per capita murder rate of 4.88 per 100,000 people puts it between Cuba at 4.72 ("reconsider travel") and Somalia at 5.56 ("do not travel").